FIGURE 2.374. Heart of the same dog-embryo, from behind. a inosculation of the vitelline veins, b left auricle, c right auricle, d auricle, e auricular ca.n.a.l, f left ventricle, g right ventricle, h arterial bulb, (From Bischoff)

FIGURE 2.375. Heart of a human embryo, four weeks old; 1. front view, 2. back view, 3. opened, and upper half of the atrium removed. a apostrophe left auricle, a double apostrophe right auricle, v apostrophe left ventricle, v double apostrophe right ventricle, ao arterial bulb, c superior vena cava (cd right, cs left), s rudiment of the interventricular wall. (From Kolliker.)

FIGURE 2.376. Heart of a human embryo, six weeks old, front view. r right ventricle, t left ventricle, s furrow between ventricles, ta arterial bulb, af furrow on its surface; to right and left are the two large auricles. (From Ecker.)

FIGURE 2.377. Heart of a human embryo, eight weeks old, back view. a apostrophe left auricle, a double apostrophe right auricle, v apostrophe left ventricle, v double apostrophe right ventricle, cd apostrophe right superior vena cava, ci inferior vena cava. (From Kolliker.))

If we compare the fully-developed arterial system of the various cla.s.ses of Craniotes, it shows a good deal of variety, yet it always proceeds from the same fundamental type. Its development is just the same in man as in the other mammals; in particular, the modification of the six pairs of arterial arches is the same in both (Figures 2.367 to 2.370). At first there is only a single pair of arches, which lie on the inner surface of the first pair of gill-arches. Behind this there then develop a second and third pair of arches (lying on the inner side of the second and third gill-arches, Figure 2.367).

Finally, we get a fourth, fifth, and sixth pair. Of the six primitive arterial arches of the Amniotes three soon pa.s.s away (the first, second, and fifth); of the remaining three, the third gives the carotids, the fourth the aortas, and the sixth (number 5 in Figures 2.364 and 2.368) the pulmonary arteries.

The human heart also develops in just the same way as that of the other mammals (Figure 2.378). We have already seen the first rudiments of its embryology, which in the main corresponds to its phylogeny (Figures 1.201 and 1.202). We saw that the palingenetic form of the heart is a spindle-shaped thickening of the gut-fibre layer in the ventral wall of the head-gut. The structure is then hollowed out, forms a simple tube, detaches from its place of origin, and henceforth lies freely in the cardiac cavity. Presently the tube bends into the shape of an S, and turns spirally on an imaginary axis in such a way that the hind part comes to lie on the dorsal surface of the fore part. The united vitelline veins open into the posterior end. From the anterior end spring the aortic arches.

(FIGURE 2.378. Heart of the adult man, fully developed, front view, natural position. a right auricle (underneath it the right ventricle), b left auricle (under it the left ventricle), C superior vena cava, V pulmonary veins, P pulmonary artery, d Botalli's duct, A aorta. (From Meyer.))

This first structure of the human heart, enclosing a very simple cavity, corresponds to the tunicate-heart, and is a reproduction of that of the Prochordonia, but it now divides into two, and subsequently into three, compartments; this reminds us for a time of the heart of the Cyclostomes and fishes. The spiral turning and bending of the heart increases, and at the same time two transverse constrictions appear, dividing it externally into three sections (Figures 2.371 and 2.372). The foremost section, which is turned towards the ventral side, and from which the aortic arches rise, reproduces the arterial bulb of the Selachii. The middle section is a simple ventricle, and the hindmost, the section turned towards the dorsal side, into which the vitelline veins inosculate, is a simple auricle (or atrium). The latter forms, like the simple atrium of the fish-heart, a pair of lateral dilatations, the auricles (Figure 2.371 b); and the constriction between the atrium and ventricle is called the auricular ca.n.a.l (Figure 2.372 ca). The heart of the human embryo is now a complete fish-heart.

(FIGURE 2.379. Transverse section of the back of the head of a chick-embryo, forty hours old. (From Kolliker.) m medulla oblongata, ph pharyngeal cavity (head-gut), h h.o.r.n.y plate, h apostrophe thicker part of it, from which the auscultory pits afterwards develop, hp skin-fibre plate, hh cervical cavity (head-coelom or cardiocoel), hzp cardiac plate (the outermost mesodermic wall of the heart), connected by the ventral mesocardium (uhg) with the gut-fibre layer or visceral coelom-layer (dfp apostrophe), Ent entoderm, ihh inner (entodermic?) wall of the heart; the two endothelial cardiac tubes are still separated by the cenogenetic septum (s) of the Amniotes, g vessels.)

In perfect harmony with its phylogeny, the embryonic development of the human heart shows a gradual transition from the fish-heart, through the amphibian and reptile, to the mammal form, The most important point in the transition is the formation of a longitudinal part.i.tion--incomplete at first, but afterwards complete--which separates all three divisions of the heart into right (venous) and left (arterial) halves (cf. Figures 2.373 to 2.378). The atrium is separated into a right and left half, each of which absorbs the corresponding auricle; into the right auricle open the body-veins (upper and lower vena cava, Figures 2.375 c and 2.377 c); the left auricle receives the pulmonary veins. In the same way a superficial interventricular furrow is soon seen in the ventricle (Figure 2.376 s). This is the external sign of the internal part.i.tion by which the ventricle is divided into two--a right venous and left arterial ventricle. Finally a longitudinal part.i.tion is formed in the third section of the primitive fish-like heart, the arterial bulb, externally indicated by a longitudinal furrow (Figure 2.376 af). The cavity of the bulb is divided into two lateral halves, the pulmonary-artery bulb, that opens into the right ventricle, and the aorta-bulb, that opens into the left ventricle. When all the part.i.tions are complete, the small (pulmonary) circulation is distinguished from the large (body) circulation; the motive centre of the former is the right half, and that of the latter the left half, of the heart.

The heart of all the Vertebrates belongs originally to the hyposoma of the head, and we accordingly find it in the embryo of man and all the other Amniotes right in front on the under-side of the head; just as in the fishes it remains permanently in front of the gullet. It afterwards descends into the trunk, with the advance in the development of the neck and breast, and at last reaches the breast, between the two lungs. At first it lies symmetrically in the middle plane of the body, so that its long axis corresponds with that of the body. In most of the mammals it remains permanently in this position.

But in the apes the axis begins to be oblique, and the apex of the heart to move towards the left side. The displacement is greatest in the anthropoid apes--chimpanzee, gorilla, and orang--which resemble man in this.

As the heart of all Vertebrates is originally, in the light of phylogeny, only a local enlargement of the middle princ.i.p.al vein, it is in perfect accord with the biogenetic law that its first structure in the embryo is a simple spindle-shaped tube in the ventral wall of the head-gut. A thin membrane, standing vertically in the middle plane, the mesocardium, connects the ventral wall of the head-gut with the lower head-wall. As the cardiac tube extends and detaches from the gut-wall, it divides the mesocardium into an upper (dorsal) and lower (ventral) plate (usually called the mesocardium anterius and posterius in man, Figure 2.379 uhg). The mesocardium divides two lateral cavities, Remak's "neck-cavities" (Figure 2.379 hh). These cavities afterwards join and form the simple pericardial cavity, and are therefore called by Kolliker the "primitive pericardial cavities."

(FIGURE 2.380. Frontal section of a human embryo, one-twelfth of an inch long in the neck, magnified forty times; "invented" by Wilhelm His. Seen from ventral side. mb mouth-fissure, surrounded by the branchial processes, ab bulbus of aorta, hm middle part of ventricle, hl left lateral part of same, ho auricle, d diaphragm, vc superior vena cava, vu umbilical vein, vo vitelline s.p.a.ce, lb liver, lg hepatic duct.)

The double cervical cavity of the Amniotes is very interesting, both from the anatomical and the evolutionary point of view; it corresponds to a part of the hyposomites of the head of the lower Vertebrates--that part of the ventral coelom-pouches which comes next to Van Wijhe's "visceral cavities" below. Each of the cavities still communicates freely behind with the two coelom-pouches of the trunk; and, just as these afterwards coalesce into a simple body-cavity (the ventral mesentery disappearing), we find the same thing happening in the head. This simple primary pericardial cavity has been well called by Gegenbaur the "head-coeloma," and by Hertwig the "pericardial breast-cavity." As it now encloses the heart, it may also be called cardiocoel.

The cardiocoel, or head-coelom, is often disproportionately large in the Amniotes, the simple cardiac tube growing considerably and lying in several folds. This causes the ventral wall of the amniote embryo, between the head and the navel, to be pushed outwards as in rupture (cf. Figure 1.180 h). A transverse fold of the ventral wall, which receives all the vein-trunks that open into the heart, grows up from below between the pericardium and the stomach, and forms a transverse part.i.tion, which is the first structure of the primary diaphragm (Figure 2.380 d). This important muscular part.i.tion, which completely separates the thoracic and abdominal cavities in the mammals alone, is still very imperfect here; the two cavities still communicate for a time by two narrow ca.n.a.ls. These ca.n.a.ls, which belong to the dorsal part of the head-coelom, and which we may call briefly pleural ducts, receive the two pulmonary sacs, which develop from the hind end of the ventral wall of the head-gut; they thus become the two pleural cavities.

The diaphragm makes its first appearance in the cla.s.s of the Amphibia (in the salamanders) as an insignificant muscular transverse fold of the ventral wall, which rises from the fore end of the transverse abdominal muscle, and grows between the pericardium and the liver. In the reptiles (tortoises and crocodiles) a later dorsal part is joined to this earlier ventral part of the rudimentary diaphragm, a pair of subvertebral muscles rising from the vertebral column and being added as "columns" to the transverse part.i.tion. But it was probably in the Permian sauro-mammals that the two originally separate parts were united, and the diaphragm became a complete part.i.tion between the thoracic and abdominal cavities in the mammals; as it considerably enlarges the chest-cavity when it contracts, it becomes an important respiratory muscle. The ontogeny of the diaphragm in man and the other mammals reproduces this phylogenetic process to-day, in accordance with the biogenetic law; in all the mammals the diaphragm is formed by the secondary conjunction of the two originally separate structures, the earlier ventral part and the later dorsal part.

Sometimes the blending of the two diaphragmatic structures, and consequently the severance of the one pleural duct from the abdominal cavity, is not completed in man. This leads to a diaphragmatic rupture (hernia diaphragmatica). The two cavities then remain in communication by an open pleural duct, and loops of the intestine may penetrate by this "rupture opening" into the chest-cavity. This is one of those fatal mis-growths that show the great part that blind chance has in organic development.

(FIGURE 2.381. Transverse section of the head of a chick-embryo, thirty-six hours old. Underneath the medullary tube the two primitive aortas (pa) can be seen in the head-plates (s) at each side of the chorda. Underneath the gullet (d) we see the aorta-end of the heart (ae), hh cervical cavity or head coelom, hk top of heart, ks head-sheath, amniotic fold, h h.o.r.n.y plate. (From Remak.)

(FIGURE 2.382. Transverse section of the cardiac region of the same chick-embryo (behind the preceding). In the cervical cavity (hh) the heart (h) is still connected by a mesocard (hg) with the gut-fibre layer (pf). d gut-gland layer, up provertebral plates, jb rudimentary auditory vesicle in the h.o.r.n.y plate, hp first rise of the amniotic fold. (From Remak.))

Thus the thoracic cavity of the mammals, with its important contents, the heart and lungs, belongs originally to the HEAD-PART of the vertebrate body, and its inclusion in the trunk is secondary. This instructive and very interesting fact is entirely proved by the concordant evidence of comparative anatomy and ontogeny. The lungs are outgrowths of the head-gut; the heart develops from its inner wall.

The pleural sacs that enclose the lungs are dorsal parts of the head-coelom, originating from the pleuroducts; the pericardium in which the heart afterwards lies is also double originally, being formed from ventral halves of the head-coelom, which only combine at a later stage. When the lung of the air-breathing Vertebrates issues from the head-cavity and enters the trunk-cavity, it follows the example of the floating bladder of the fishes, which also originates from the pharyngeal wall in the shape of a small pouch-like out-growth, but soon grows so large that, in order to find room, it has to pa.s.s far behind into the trunk-cavity. To put it more precisely, the lung of the quadrupeds retains this hereditary growth-process of the fishes; for the hydrostatic floating bladder of the latter is the air-filled organ from which the air-breathing organ of the former has been evolved.

There is an interesting cenogenetic phenomenon in the formation of the heart of the higher Vertebrates that deserves special notice. In its earliest form the heart is DOUBLE, as recent observation has shown, in all the Amniotes, and the simple spindle-shaped cardiac tube, which we took as our starting-point, is only formed at a later stage, when the two lateral tubes move backwards, touch each other, and at last combine in the middle line. In man, as in the rabbit, the two embryonic hearts are still far apart at the stage when there are already eight primitive segments (Figure 1.134 h). So also the two coelom-pouches of the head in which they lie are still separated by a broad s.p.a.ce. It is not until the permanent body of the embryo develops and detaches from the embryonic vesicle that the separate lateral structures join together, and finally combine in the middle line. As the median part.i.tion between the right and left cardiocoel disappears, the two cervical cavities freely communicate (Figure 2.381), and form, on the ventral side of the amniote head, a horseshoe-shaped arch, the points of which advance backwards into the pleuro-ducts or pleural cavities, and from there into the two peritoneal sacs of the trunk.

But even after the conjunction of the cervical cavities (Figure 2.381) the two cardiac tubes remain separate at first; and even after they have united a delicate part.i.tion in the middle of the simple endothelial tube (Figures 2.379 s and 2.382 h) indicates the original separation. This CENOGENETIC "primary cardiac septum" presently disappears, and has no relation to the subsequent permanent part.i.tion between the halves of the heart, which, as a heritage from the reptiles, has a great PALINGENETIC importance.

Thorough opponents of the biogenetic law have laid great stress on these and similar cenogenetic phenomena, and endeavoured to urge them as striking disproofs of the law. As in every other instance, careful, discriminating, comparative-morphological examination converts these supposed disproofs of evolution into strong arguments in its favour.

In his excellent work, On the structure of the Heart in the Amphibia (1886), Carl Rabl has shown how easily these curious cenogenetic facts can be explained by the secondary adaptation of the embryonic structure to the great extension of the food-yelk.

The embryology of all the other parts of the vascular system also gives us abundant and valuable data for the purposes of phylogeny. But as one needs a thorough knowledge of the intricate structure of the whole vascular system in man and the other Vertebrates in order to follow this with profit, we cannot go into it further here. Moreover, many important features in the ontogeny of the vascular system are still very obscure and controverted. The characters of the embryonic circulation of the Amniotes, which we have previously considered (Chapter 1.15), are late acquisitions and entirely cenogenetic. (Cf.

Chapter 1.15 and Figures 1.198 to 1.202.)

In the Selachii also we find a longitudinal row of segmental ca.n.a.ls on each side, which open outwards into the primitive renal ducts (nephrotomes, Chapter 1.14). The segmental ca.n.a.ls (a pair in each segment of the middle part of the body) open internally by a ciliated funnel into the body-cavity. From the posterior group of these organs a compact primitive kidney is formed, the anterior group taking part in the construction of the s.e.xual organs.

In the same simple form that remains throughout life in the Myxinoides and partly in the Selachii we find the primitive kidney first developing in the embryo of man and the higher Craniotes (Figures 2.386 and 2.387). Of the two parts that compose the comb-shaped primitive kidney the longitudinal channel, or nephroduct, is always the first to appear; afterwards the transverse "ca.n.a.ls," the excreting nephridia, are formed in the mesoderm; and after this again the Malpighian capsules with their arterial coils are a.s.sociated with these as coelous outgrowths. The primitive renal duct, which appears first, is found in all craniote embryos at the early stage in which the differentiation of the medullary tube takes place in the ectoderm, the severance of the chorda from the visceral layer in the entoderm, and the first trace of the coelom-pouches arises between the limiting layers (Figure 2.385). The nephroduct (ung) is seen on each side, directly under the h.o.r.n.y plate, in the shape of a long, thin, thread-like string of cells. It presently hollows out and becomes a ca.n.a.l, running straight from front to back, and clearly showing in the transverse section of the embryo its original position in the s.p.a.ce between h.o.r.n.y plate (h), primitive segments (uw), and lateral plates (hpl). As the originally very short urinary ca.n.a.ls lengthen and multiply, each of the two primitive kidneys a.s.sumes the form of a half-feathered leaf (Figure 2.387). The lines of the leaf are represented by the urinary ca.n.a.ls (u), and the rib by the outlying nephroduct (w). At the inner edge of the primitive kidneys the rudiment of the ventral s.e.xual gland (g) can now be seen as a body of some size. The hindermost end of the nephroduct opens right behind into the last section of the r.e.c.t.u.m, thus making a cloaca of it.

However, this opening of the nephroducts into the intestine must be regarded as a secondary formation. Originally they open, as the Cyclostomes clearly show, quite independently of the gut, in the external skin of the abdomen.

(FIGURE 2.395. Primitive kidneys and germinal glands of a human embryo, three inches in length (beginning of the sixth week), magnified fifteen times. k germinal gland, u primitive kidney, z diaphragmatic ligament of same, w Wolffian duct (opened on the right), g directing ligament (gubernaculum), a allantoic duct. (From Kollmann.))

In the Myxinoides the primitive kidneys retain this simple comb-shaped structure, and a part of it is preserved in the Selachii; but in all the other Craniotes it is only found for a short time in the embryo, as an ontogenetic reproduction of the earlier phylogenetic structure.

In these the primitive kidney soon a.s.sumes the form (by the rapid growth, lengthening, increase, and serpentining of the urinary ca.n.a.ls) of a large compact gland, of a long, oval or spindle-shaped character, which pa.s.ses through the greater part of the embryonic body-cavity (Figures 1.183 m, 1.184 m, 2.388 n). It lies near the middle line, directly under the primitive vertebral column, and reaches from the cardiac region to the cloaca. The right and left kidneys are parallel to each other, quite close together, and only separated by the mesentery--the thin narrow layer that attaches the middle gut to the under surface of the vertebral column. The pa.s.sage of each primitive kidney, the nephroduct, runs towards the back on the lower and outer side of the gland, and opens in the cloaca, close to the starting-point of the allantois; it afterwards opens into the allantois itself.

(FIGURES 2.396 TO 2.398. Urinary and s.e.xual organs of ox-embryos.

Figure 2.396, female embryo one and a half inches long; Figure 2.397, male embryo, one and a half inches long. Figure 2.398 female embryo two and a half inches long. w primitive kidney, wg Wolffian duct, m Mullerian duct, m apostrophe upper end of same (opened at t), i lower and thicker part of same (rudiment of uterus), g genital cord, h t.e.s.t.i.c.l.e, (h apostrophe, lower and h double apostrophe, upper testicular ligament), o ovary, o apostrophe lower ovarian ligament, i inguinal ligament of primitive kidney, d diaphragmatic ligament of primitive kidney, nn accessory kidneys, n permanent kidneys, under them the S-shaped ureters, between these the r.e.c.t.u.m, v bladder, a umbilical artery. (From Kolliker.))

The primitive or primordial kidneys of the amniote embryo were formerly called the "Wolffian bodies," and sometimes "Oken's bodies."

They act for a time as kidneys, absorbing unusable juices from the embryonic body and conducting them to the cloaca--afterwards to the allantois. There the primitive urine acc.u.mulates, and thus the allantois acts as bladder or urinary sac in the embryos of man and the other Amniotes. It has, however, no genetic connection with the primitive kidneys, but is a pouch-like growth from the anterior wall of the r.e.c.t.u.m (Figure 1.147 u). Thus it is a product of the visceral layer, whereas the primitive kidneys are a product of the middle layer. Phylogenetically we must suppose that the allantois originated as a pouch-like growth from the cloaca-wall in consequence of the expansion caused by the urine acc.u.mulated in it and excreted by the kidneys. It is originally a blind sac of the r.e.c.t.u.m. The real bladder of the vertebrate certainly made its first appearance among the Dipneusts (in Lepidosiren), and has been transmitted from them to the Amphibia, and from these to the Amniotes. In the embryo of the latter it protrudes far out of the not yet closed ventral wall. It is true that many of the fishes also have a "bladder." But this is merely a local enlargement of the lower section of the nephroducts, and so totally different in origin and composition from the real bladder. The two structures can be compared from the physiological point of view, and so are a.n.a.lOGOUS, as they have the same function; but not from the morphological point of view, and are therefore not h.o.m.oLOGOUS. The false bladder of the fishes is a mesodermic product of the nephroducts; the true bladder of the Dipneusts, Amphibia, and Amniotes is an entodermic blind sac of the r.e.c.t.u.m.

In all the Anamnia (the lower amnionless Craniotes, Cyclostomes, Fishes, Dipneusts, and Amphibia) the urinary organs remain at a lower stage of development to this extent, that the primitive kidneys (protonephri) act permanently as urinary glands. This is only so as a pa.s.sing phase of the early embryonic life in the three higher cla.s.ses of Vertebrates, the Amniotes. In these the permanent or after or secondary (really tertiary) kidneys (renes or metanephri) that are distinctive of these three cla.s.ses soon make their appearance. They represent the third and last generation of the vertebrate kidneys. The permanent kidneys do not arise (as was long supposed) as independent glands from the alimentary tube, but from the last section of the primitive kidneys and the nephroduct. Here a simple tube, the secondary renal duct, develops, near the point of its entry into the cloaca; and this tube grows considerably forward. With its blind upper or anterior end is connected a glandular renal growth, that owes its origin to a differentiation of the last part of the primitive kidneys.

This rudiment of the permanent kidneys consists of coiled urinary ca.n.a.ls with Malpighian capsules and vascular coils (without ciliated funnels), of the same structure as the segmental mesonephridia of the primitive kidneys. The further growth of these metanephridia gives rise to the compact permanent kidneys, which have the familiar bean-shape in man and most of the higher mammals, but consist of a number of separate folds in the lower mammals, birds, and reptiles. As the permanent kidneys grow rapidly and advance forward, their pa.s.sage, the ureter, detaches altogether from its birth-place, the posterior end of the nephroduct; it pa.s.ses to the posterior surface of the allantois. At first in the oldest Amniotes this ureter opens into the cloaca together with the last section of the nephroduct, but afterwards separately from this, and finally into the permanent bladder apart from the r.e.c.t.u.m altogether. The bladder originates from the hindmost and lowest part of the allantoic pedicle (urachus), which enlarges in spindle shape before the entry into the cloaca. The anterior or upper part of the pedicle, which runs to the navel in the ventral wall of the embryo, atrophies subsequently, and only a useless string-like relic of it is left as a rudimentary organ; that is the single vesico-umbilical ligament. To the right and left of it in the adult male are a couple of other rudimentary organs, the lateral vesico-umbilical ligaments. These are the degenerate string-like relics of the earlier umbilical arteries.

Though in man and all the other Amniotes the primitive kidneys are thus early replaced by the permanent kidneys, and these alone then act as urinary organs, all the parts of the former are by no means lost.

The nephroducts become very important physiologically by being converted into the pa.s.sages of the s.e.xual glands. In all the Gnathostomes--or all the Vertebrates from the fishes up to man--a second similar ca.n.a.l develops beside the nephroduct at an early stage of embryonic evolution. The latter is usually called the Mullerian duct, after its discoverer, Johannes Muller, while the former is called the Wolffian duct. The origin of the Mullerian duct is still obscure; comparative anatomy and ontogeny seem to indicate that it originates by differentiation from the Wolffian duct. Perhaps it would be best to say: "The original primary nephroduct divides by differentiation (or longitudinal cleavage) into two secondary nephroducts, the Wolffian and the Mullerian ducts." The latter (Figure 2.387 m) lies just on the inner side of the former (Figure 2.387 w).

Both open behind into the cloaca.

However uncertain the origin of the nephroduct and its two products, the Mullerian and the Wolffian ducts, may be, its later development is clear enough. In all the Gnathostomes the Wolffian duct is converted into the spermaduct, and the Mullerian duct into the oviduct. Only one of them is retained in each s.e.x; the other either disappears altogether, or only leaves relics in the shape of rudimentary organs.

In the male s.e.x, in which the two Wolffian ducts become the spermaducts, we often find traces of the Mullerian ducts, which I have called "Rathke's ca.n.a.ls" (Figure 2.394 c). In the female s.e.x, in which the two Mullerian ducts form the oviducts, there are relics of the Wolffian ducts, which are called "the ducts of Gaertner."

(FIGURE 2.399. Female s.e.xual organs of a Monotreme (Ornithorhynchus, Figure 2.269). o ovaries, t oviducts, u womb, sug urogenital sinus; at u apostrophe is the outlet of the two wombs, and between them the bladder (vu). cl cloaca. (From Gegenbaur.)

FIGURES 2.400 AND 2.401. Original position of the s.e.xual glands in the ventral cavity of the human embryo (three months old).

FIGURE 2.400 male (natural size). h t.e.s.t.i.c.l.es, gh conducting ligament of the t.e.s.t.i.c.l.es, wg spermaduct, h bladder, uh inferior vena cava, nn accessory kidneys, n kidneys.

FIGURE 2.401 female, slightly magnified. r round maternal ligament (underneath it the bladder, over it the ovaries). r apostrophe kidneys, s accessory kidneys, c caec.u.m, o small reticle, om large reticle (stomach between the two), l spleen. (From Kolliker.))

We obtain the most interesting information with regard to this remarkable evolution of the nephroducts and their a.s.sociation with the s.e.xual glands from the Amphibia (Figures 2.390 to 2.395). The first structure of the nephroduct and its differentiation into Mullerian and Wolffian ducts are just the same in both s.e.xes in the Amphibia, as in the mammal embryos (Figures 2.392 and 2.396). In the female Amphibia the Mullerian duct develops on either side into a large oviduct (Figure 2.393 od), while the Wolffian duct acts permanently as ureter (u). In the male Amphibia the Mullerian duct only remains as a rudimentary organ without any functional significance, as Rathke's ca.n.a.l (Figure 2.394 c); the Wolffian duct serves also as ureter, but at the same time as spermaduct, the sperm-ca.n.a.ls (ve) that proceed from the t.e.s.t.i.c.l.es (t) entering the fore part of the primitive kidneys and combining there with the urinary ca.n.a.ls.

In the mammals these permanent amphibian features are only seen as brief phases of the earlier period of embryonic development (Figure 2.392). Here the primitive kidneys, which act as excretory organs of urine throughout life in the amnion-less Vertebrates, are replaced in the mammals by the permanent kidneys. The real primitive kidneys disappear for the most part at an early stage of development, and only small relics of them remain. In the male mammal the epididymis develops from the uppermost part of the primitive kidney; in the female a useless rudimentary organ, the epovarium, is formed from the same part. The atrophied relic of the former is known as the paradidymis, that of the latter as the parovarium.

(FIGURE 2.402. Urogenital system of a human embryo of three inches in length, double natural size. h t.e.s.t.i.c.l.es, wg spermaducts, gh conducting ligament, p processus v.a.g.i.n.alis, b bladder, au umbilical arteries, m mesorchium, d intestine, u ureter, n kidney, nn accessory kidney. (From Kollman.))

The Mullerian ducts undergo very important changes in the female mammal. The oviducts proper are developed only from their upper part; the lower part dilates into a spindle-shaped tube with thick muscular wall, in which the impregnated ovum develops into the embryo. This is the womb (uterus). At first the two wombs (Figure 2.399 u) are completely separate, and open into the cloaca on either side of the bladder (vu), as is still the case in the lowest living mammals, the Monotremes. But in the Marsupials a communication is opened between the two Mullerian ducts, and in the Placentals they combine below with the rudimentary Wolffian ducts to form a single "genital cord." The original independence of the two wombs and the v.a.g.i.n.al ca.n.a.ls formed from their lower ends are retained in many of the lower Placentals, but in the higher they gradually blend and form a single organ. The conjunction proceeds from below (or behind) upwards (or forwards). In many of the Rodents (such as the rabbit and squirrel) two separate wombs still open into the simple and single v.a.g.i.n.al ca.n.a.l; but in others, and in the Carnivora, Cetacea, and Ungulates, the lower halves of the wombs have already fused into a single piece, though the upper halves (or "horns") are still separate ("two-horned" womb, uteris bicornis). In the bats and lemurs the "horns" are very short, and the lower common part is longer. Finally, in the apes and in man the blending of the two halves is complete, and there is only the one simple, pear-shaped uterine pouch, into which the oviducts open on each side. This simple uterus is a late evolutionary product, and is found ONLY in the ape and man.

(FIGURES 2.403 TO 2.406. Origin of human ova in the female ovary.

FIGURE 2.403. Vertical section of the ovary of a new-born female infant, a ovarian epithelium, b rudimentary string of ova, c young ova in the epithelium, d long string of ova with follicle-formation (Pfluger's tube), e group of young follicles, f isolated young follicle, g blood-vessels in connective tissue (stroma) of the ovary.

In the strings the young ova are distinguished by their considerable size from the surrounding follicle-cells. (From Waldeyer.)