(b) Certain tubuli described by Weiss as situated in a series along the upper corners of the atrial cavity, and communicating, after the fashion, of the "nephridia" of the earthworm, with the coelom and with the exterior (or, rather, with that portion of the animal's exterior enclosed in by the atrial wall; compare Section 6).

(c) The general epithelial lining of the atrium.

The reproductive organs (Figure 4, Sheet 20, g.) are ma.s.ses of cells situated in an isolated part of the coelom in the atrial folds, and, having no ducts, their contents must escape into the atrium by rupture of the body-wall. Thence they escape either by gill-slits, pharynx and mouth, or, more generally, through the atrial pore. The animals, like all the vertebrata, are dioecious, i.e., male or female.

Section 10. The endostyle (end.), in Figures 3 and 4, is a ciliated path or groove on the under side of the pharynx, which is generally supposed to represent the thyroid gland of vertebrates. The vertebrate thyroid, early in development, is certainly an open and long narrow groove in the ventral side of the pharynx. The hyper-pharyngeal groove (h.p.) has been in the past compared to the pituitary body, but there is little doubt now that this structure is represented by the ciliated pit.

Section 11. The student is advised to revise this chapter before proceeding, and to schedule carefully the anatomical features under the headings of (1.) distinctly vertebrate characters, (2.) characters contrasting with the normal vertebrate structure, (3.) facts of doubtful import, with the suggestions given in the text written against them.

2. _The Development of Amphioxus_

Section 12. The development of amphioxus, studied completely, is at once one of the most alluring and difficult tasks in the way of the zoologist; but certain of its earlier and most obvious fasts may very conveniently be taken into consideration now.

Section 13. The phenomena of the extrusion of polar bodies and fertilization are treated of later, and will, therefore, not be considered now. We will start our description with an egg-cell, which has escaped, of course, since there are no genital ducts, by rupture of the parent, has been fertilized by the male element, and is about to develop into a young amphioxus. It is simply a single cell, with some power of amoeboid motion, a single nucleus and nucleolus; and in amphioxus its protoplasm is clear and transparent. Frequently ova are loaded with granules of food store (yolk), which enable the young animal to go far with its development before it is hatched and has to begin fending for itself. Such an ovum as that of our present type, however being devoid of such yolk (alecithal = without yolk), necessitates a very early start in life, and, for reasons too complicated to state fully here, the development in such a case is considered particularly instructive and primitive by zoologists.

Section 14. The first thing to be seen in the developing cell is a deepening circular groove (Figure 1, Sheet 21), which divides the ovum into two parts. Another groove then cuts at right angles to this subdividing the two into four (Figure 2). Another groove, at right angles to both the former, follows, making the four eight (Figure 3). And so subdivision goes on. The whole process is called segmentation or cleavage.

Section 15. At the end of segmentation we get a hollow sphere of small cells, the cells separating from one another centrally and enclosing a cavity as the process proceeds. This is the blastosphere, shown diagrammatically in Figure 4, and of which an internal view, rather truer to the facts of the case as regards shape, is given as Figure 5. The central cavity is the segmentation cavity (s.c.).

Section 16. Inv.a.g.i.n.ation follows (Figure 6). In this process a portion of the blastosphere wall is the tucked into the rest, as indicated by the arrow, so that a two-layered sack is formed. The s.p.a.ce ar. is the archenteron, the primordial intestine, and its mouth is called, the blastopore (bp.). The outer layer of this double-walled sac is called the epiblast. For the present we will give the inner lining no special term. The young amphioxus has, at this stage, which is called the gastrula stage, a curious parallelism with such a lowly form as the Hydra of our ditches. This latter creature, like the gastrula, consists essentially of two layers of cells, an outer protective and sensory layer, and an inner digestive one; it has a primordial intestine, or archenteron, and its mouth is sometimes regarded as being a blastopore. All animals that have little yolk, and start early in life for themselves, pa.s.s through a gastrula stage, substantially the same as this of amphioxus.

Section 17. The a.n.u.s is perforated later near the region occupied at this stage by the blastopore. Hence the anterior end of the future amphioxus, the head end, is pointing towards the Figure 6, and the letters ep. are marked on the side which will be dorsal.

Section 18. Figure 7 i. is a dorsal view of the gastrula at a somewhat later stage, and here indications of distinctly vertebrate relationships already appear. Figure 7 ii. is a cross-section, its position, being shown by cross-lines in 7 i. and 6. Note first that the epiblast along the mid-dorsal line is sinking in to form what is called the neural plate (n.p.), and simultaneously on either side of it rise the neural folds (n.f.). Now, at Figure 8, a slightly later stage is represented, and at 9 i. the inturned part is separated from the general external epiblast as the spinal cord. The remainder of the epiblast const.i.tutes the epidermis.

Section 19. Reverting to Figure 7 ii., along the dorsal side of the archenteron a thickening of its wall appears, and is gradually pinched off from it to form a cellular rod, lying along under the nervous axis and above the intestine. This is the notochord (compare Figures 8 and 9).

Section 20. Finally, we note two series of buds of cells, one on either side of the archenteron in Figure 7 ii. In 8 these buds have become hollow vesicles, growing out from it, the coelomic pouches. They are further developed in 9; and in 9 ii., which is a diagrammatic figure, they are indicated by dotted lines. They finally appear to (? entirely) obliterate the segmentation cavity-- they certainly do so throughout the body; and their cavities are in time cut off from the mesenteron, by the gradual constriction of their openings. In this way the coelom (body cavity) arises as a series of hollow "archenteric" outgrowths, and ms. becomes the alimentary ca.n.a.l. mt.c., the metapleural ca.n.a.ls, probably arise subsequently to, and independently of, the general coelomic s.p.a.ce, by a splitting in the body-wall substance.

Section 21. Hence, in considering the structure of amphioxus, we have three series of cells from which its tissues are developed:--

1. The epiblast.

2. Walls of the coelomic pouches, which form (a) an inner lining to the epiblast, (b) an outer coating to the hypoblast, and (c) the mesentery (m.), by which the intestine is supported. This is the mesoblast.

3. The lining of the mesenteron, or hypoblast.

From the epiblast the epidermis (not the dermis), the nervous system (including the nerves), and the sensory part of all sense organs are derived. From the mesoblast the muscles, the dermis genital and excretory organs, circulatory fluid and apparatus, any skeletal structures; and all connective tissue are derived. The ma.s.s of the body is thus evidently made of mesoblast. The hypoblast is the lining of the intestine and of the glands which open into it; and the material of the notochord is also regarded, as hypoblast.

Section 22. Figure 9 ii. shows all the essential points of the structure of amphioxus. Epiblast is indicated by a line of dashes, mesoblast by dots, and hypoblast, dark or black. The true mouth is formed late by a tucking-in of epiblast, the stomodaeum (s.d.), which meets and fuses with the hypoblast, and is then perforated. The position of this mouth is at the velum. The formation of the atrium has been described. The metapleural folds run forward in front of the velum, as the epipleurs (ep. in Sections 1 and 2), and form an oral hood (b.c.), around which the tentacles appear, and which is evidently not equivalent to the vertebrate mouth cavity, but in front of and outside it. The a.n.u.s is formed by a tucking in, the proctodaeum, similar to the stomodaeum.

Section 23. The formation of the respiratory slits is complicated, and difficult to describe, but, since investigators have still to render its meaning apparent, it need not detain the elementary student.*

* See Balfour's Embryology, Volume 2, and Quarterly Journal of Microscopical Science March, 1891.

_Questions on Amphioxus_

1. Draw diagrams, with the parts named, of the alimentary ca.n.a.l of (a) amphioxus, (b) any craniate; (c) indicate very shortly the princ.i.p.al structural differences between the two.

2. Describe, with a diagram, the circulation of amphioxus. Compare it with that of the craniata.

3. Draw from memory transverse sections, of amphioxus (a) in the oral region, (b) through the pharynx, (c) just anterior, and (d) just posterior to atrial pore.

4. Describe fully the coelom of amphioxus, and compare it with that of the frog in regard to (a) development, (b) its relation to other organs in the adult.

5. Compare the atrial cavity and coelom of amphioxus. To what series of cavities in the frog are the metapleural ca.n.a.ls to be compared?

6. Describe the notochord of amphioxus, and point out its differences from the vertebrate notochord.

7. Describe, with diagrams, the nervous system of amphioxus, and compare its nervous axis, in detail, with that of a vertebrate.

8. Compare the genital organs of amphioxus with those of a higher vertebrate.

9. What structures have been regarded, as renal organs in amphioxus?

10. What is a gastrula? With what lower type has the gastrula been compared? Discuss the comparison.

-Development_

_The Development of the Frog_

Section 1. We have now to consider how the body of the frog is built up out of the egg cell, but previously to doing so we must revert to the reproductive organs of our type.

Section 2. In the testes of the male is found an intricate network of tubuli, the lining of which is, of course, an epithelium. The cells of this epithelium have their internal borders differentiated into spermatozoa, which, at a subsequent stage, are liberated. A spermatozoon (Figure 3, Sheet 13, sp.) is a rod-shaped cell containing a nucleus; in fact, consisting chiefly of nucleus, with a tail, the flagellum, which is vibratile, and forces the spermatozoon, forward by its lashing. The spermatozoa float in a fluid which is the joint product of the testes, anterior part of the kidney, and perhaps the prostate glands.

Section 3. In the ovary, the ova are formed, and grow to a considerable size. They are nucleated cells, the nucleus going by the special name of the germinal vesicle and the nucleolus the germinal spot. The ova prey upon the adjacent cells as they develop.

The protoplasm of the ovum, except at that part of the surface where the germinal vesicle lies, is packed with a great amount of food material, the yolk granules. This yolk is non-living inert matter. An ovum such as this, in which the protoplasm is concentrated towards one pole, is called telolecithal.