=Each Cell bears in itself Ancestral Energy.= As we have already seen, the germinal, cells are not the only ones which possess the energies of all the characters of the species. On the contrary it becomes more and more certain, from further investigation, that each cell of the body bears in itself, so to speak, all the energies of the species, as is distinctly seen in plants. But in all the cells which are not capable of germinating, these energies remain incapable of development. It results that such energies, remaining virtual, have no practical importance.

In an a.n.a.logous sense we may say that all the cells of the body are hermaphrodite, as all germinal cells, for each possesses in itself the undifferentiated energies of each s.e.x. Each spermatozoid contains all the energies of the paternal and maternal ancestry of man, and each egg those of the paternal and maternal ancestry of woman. The male and the female are only the bearers of each kind of germinal cells necessary for conjugation, and each of these bearers only differs from the others by its s.e.xual cells and by what is called correlative s.e.xual differences. But we must not forget that the germinal cells themselves are only differentiated at a certain period in the development of the embryo; they are thus hermaphrodite originally and only become male and female later.

New experiments made on the eggs of sea urchins and other organisms have shown that conjugation may be replaced by an external irritating agent; for example, the action of certain chemical substances is sufficient to make eggs develop by parthenogenesis which would have died without this action. An entire being has been successfully produced from an egg divided into two by means of a hair. And even from the protoplasm of the egg without its nucleus, with the aid of a spermatozoid. We must not, however, base premature hypotheses on these facts.

When a female cell, or egg, develops without fecundation (parthenogenesis) its nucleus enlarges and divides in the same manner as conjugated nuclei (mitosis).

A point of general interest is what is called the _specific polyembryony_ of certain parasitic insects (hymenoptera of the genus _Encyrtus_). According to _Marchal_, their eggs grow and divide into a considerable number of secondary eggs, each of which gives rise to an embryo and later on a perfect insect. By shaking the eggs of certain marine animals they have been caused to divide into several eggs and thus to produce several embryos. All the individuals arising from the division of the same egg of _Encyrtus_ are of the same s.e.x.

[Ill.u.s.tration: PLATE I

CELL DIVISION

FIG. 1. Cell before division.

FIG. 2. Division of centrosome.

FIG. 3. Formation of chromosomes.

FIG. 4. Dissolution of nucleus.

FIG. 5. Lining up of chromosomes.

FIG. 6. Division of chromosomes.

FIG. 7. Division of chromosomes.

FIG. 8. Attraction of chromosomes by centrosomes.

FIG. 9. Concentration of nuclei. Division of cell.

FIG. 10. Formation of new chromatin.]

[Ill.u.s.tration: PLATE II

FERTILIZATION OF THE OVUM BY THE SPERMATOZOID

DIAGRAM OF OVUM AND SPERMATOZOID

FIG. 11. _a_, Vitelline membrane; _b_, protoplasm, or vitellus; _c_, nucleus with chromatin; _d_, spermatozoid penetrating egg; _e_, another spermatozoid arrested by the vitelline membrane.

FIG. 12. Formation of centrosome.

FIG. 13. Formation of male nucleus by spermatozoid. Division of centrosome.

FIG. 14. Development of nucleus of spermatozoid.

FIG. 15. Nucleus of spermatozoid attains same size as that of ovum.

FIG. 16. Formation of male and female chromosomes.

FIG. 17. Lining up of male and female chromosomes.]

=Embryology.=--It is not necessary to describe here in detail the different changes which the two conjugated cells pa.s.s through to become an adult man. This is the object of the science of embryology.

We shall return to this in Chapter III. A few words are necessary, however, to explain the general principles.

=Ovulation. The corpus luteum.=--The ovaries of woman (Fig. 18) contain a considerable number of cells or ovules, although infinitely less than the number of spermatozoids contained in the t.e.s.t.i.c.l.es. From time to time some of these ovules enlarge and are surrounded by a vesicle with liquid contents, which is called the Graafian follicle.

At the time of the monthly periods an egg (sometimes two) is discharged from its Graafian follicle, from one or other ovary. This phenomenon is called _ovulation_. The empty follicle becomes cicatrized in the ovary and is called the _corpus luteum_ (yellow body).

The egg after its discharge arrives at the abdominal orifice of the Fallopian tube, which communicates directly with the abdominal cavity.

Some authors state that the end of the tube becomes applied against the ovary by the aid of muscular movement and, so to speak, sucks in the discharged ovule, while others hold that the movements of the vibratile cilia, with which the epithelium of the tubes is furnished, suffice to draw the ovule into its cavity. Figure 18 explains this phenomenon.

Having arrived in the tube, the ovule moves very slowly in the almost capillary tube by means of the vibratile cilia and arrives in the cavity of the womb. Fecundation probably takes place most often at the entrance to the tube or in its ca.n.a.l; sometimes possibly in the womb.

On some occasions a squad of spermatozoids advances to meet the descending egg, and numerous spermatozoids are often found in the tubes, even as far as the abdominal cavity.

=Fixation of the egg. Formation of the Decidua.=--After fecundation, the egg becomes attached to the mucous membrane of the cavity of the womb. This mucous membrane proliferates and becomes gradually detached from the womb to form the _membrana decidua_ which envelops the egg or ovule. An egg fecundated and fixed in this way may keep its position and grow during the first weeks of pregnancy, by the aid of villosities covering its envelope which penetrate the wall of the womb.

[Ill.u.s.tration: FIG. 18. Diagrammatic section in median plane of the female genital organs. It shows the position of an ovule which has just been discharged lying in the opening of the right tube, and that of another ovary fecundated and surrounded by the decidual membrane. In reality this could hardly coexist with the other ovule freely discharged. In the right ovary are seen ovules in various degrees of maturity in their Graafian follicles: also a corpus luteum--an empty Graafian follicle after expulsion of the ovule. The figure also shows the end of the p.e.n.i.s in the v.a.g.i.n.a at the moment of e.j.a.c.u.l.a.t.i.o.n of s.e.m.e.n, and the position of a preventive to avoid fecundation.]

[Ill.u.s.tration: FIG. 19. The mouth of the tube applied to the ovary at the moment of expulsion of the ovule.]

=The womb. The placenta.= The womb or uterus is the size of a small egg flattened in one direction. It terminates below in the neck or _cervix_, which is prolonged into the v.a.g.i.n.a as a projection, called the v.a.g.i.n.al portion of the uterus. The cavity of the womb is continued into the neck and opens below in the v.a.g.i.n.a by an aperture which is round in virgins and is called the external _os uteri_. The walls of the womb consist of a thick layer of unstriped muscle. When childbirth takes place it causes tearing which makes the external os uteri irregular and fissured. During copulation the aperture of the p.e.n.i.s or male organ is placed nearly opposite the os uteri, which facilitates the entrance of spermatozoa into the uterus. (For the ill.u.s.tration of these points see Fig. 18.)

The vitellus and the membrane of the egg enlarge with the embryo and absorb by endosmosis the nutritive matter necessary for the latter, contained in the maternal blood. The womb itself enlarges at the same time as the embryo.

[Ill.u.s.tration: FIG. 20. Human egg of the second week: magnified eight times. (After _Kolliker_.) _Chor._ Chorion or envelope of the egg.

_Vill._ Villi of the chorion.

_Emb._ Embryo (near the head are seen the branchial arches).

_Umb._ Umbilical vesicle.

_Am._ Amnion.]

The fasciculus attached to the embryo is the allantois which becomes the umbilical cord. The vertebrae are already easy to recognize in this embryo. The embryo is formed from a portion of blastoderm, that is to say, from the cellular layer applied to the membranes of the egg and arising from the successive divisions of the two primary conjugated cells and their daughter cells. The embryo has the form of a spatula with the head at one end and the tail at the other. From its walls is detached a surrounding vesicle (Fig. 20) called the _amnion_, while another vesicle, the _umbilical vesicle_, grows from its ventral surface and serves, in birds, for the vitelline circulation of the egg which is detached from the mother's body.

In man, the umbilical vesicle is unimportant. In its place the circulation of the blood takes place by the aid of another vesicle, called the _allantois_, which arises from the intestine of the embryo, and which becomes attached to the walls of the womb in the form of a thick disk called the placenta.

[Ill.u.s.tration: FIG. 21. Embryo of four weeks (After _Kolliker_).

1. Auditory vesicle.

2. Ocular vesicle.

3. Olfactory fossa.

4. Bud forming upper maxilla.

5. Bud of lower maxilla.

6. Right ear.

7. Liver.

8. Upper limb.

9. Lower limb.

10. Caudal extremity.]

The placenta is formed of dilated blood vessels which meet the maternal blood vessels, also dilated, in the uterine wall, allantois later on becomes the umbilical cord.

In the placenta the embryonic and maternal vessels without actually communicating, are placed in intimate contact, which allows nutritive matter and oxygen to pa.s.s by endosmosis from the maternal vessels to those of the embryo. Figure 21 shows a human embryo at the beginning of the fifth week of pregnancy.

[Ill.u.s.tration: FIG. 22. Sagittal section of a primipara in the last month of pregnancy.]

=Duration of pregnancy. Birth.= Pregnancy lasts from conjugation, which is synonymous with conception, till birth, that is about nine months (ten lunar months of four weeks). The embryo is then ready to separate from the maternal body (Fig. 22). By the act of birth it is expelled violently, bringing with it the umbilical cord and the placenta (Fig. 23). Immediately afterward the empty womb contracts strongly and gradually recovers its former size. The sudden interruption of its communications with the maternal circulation deprives the embryo, which has suddenly become a child, of its nutritive matter and oxygen.

[Ill.u.s.tration: FIG. 23. Sagittal section of frozen body of a woman in labor: the head of the child is engaged in the neck of the womb; the orifice of the neck of the womb (_os uteri_) is already fully dilated and the bag of waters commences to project from the v.u.l.v.a: it is formed by the former membranes of the egg and the decidua.]

In order to avoid suffocation it is obliged to breathe atmospheric air immediately, for its blood becomes dark by saturation with carbonic acid, which irritates the respiratory nerve centers. The first independent act of the new-born child is, therefore, a nervous reflex determined by asphyxia, and is performed with the first cry. Soon afterward the infant begins to suck, so as not to die of hunger, while the umbilical cord, having become useless, shrivels up, and the placenta is destroyed (some animals eat it). The new-born infant is only distinguished from the embryo soon after birth by its breathing and crying.

We may, therefore, say that infancy, especially early infancy, is only a continuation of embryonic life. The transformations which the infant undergoes from birth to adult age are known to all. They take place more and more slowly, except at the relatively short period of p.u.b.erty.

=Formation of the s.e.xual glands.=--We must remember that at a very early embryonic period certain groups of cells are reserved to form later on the s.e.xual glands. These cells are at first neither male nor female, but are undifferentiated; later on they become differentiated to form in certain individuals, called males, the t.e.s.t.i.c.l.es with their spermatozoa, and in others, called females, the ovaries with their eggs. On this differentiation depends the s.e.x of the individual, and, according as it takes place in one way or the other, all the rest of the body develops with the correlative s.e.xual characters of the corresponding s.e.x (at first the external genital organs peculiar to each s.e.x, then the beard in man, the b.r.e.a.s.t.s in woman, etc).

=Castration. Correlative s.e.xual characters.=--Castration is the term applied to the extirpation of the s.e.xual glands. When it takes place in infancy it causes a considerable change in the whole subsequent development of the body, especially in man, but also in woman. Man becomes more slender, preserves a high and infantile voice, and his s.e.xual correlative characters develop incompletely or not at all.