In an analogous 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 sex. 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 sexual cells and by what is called correlative sexual 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 sex.
PLATE I
CELL DIVISION
PLATE II
FERTILIZATION OF THE OVUM BY THE SPERMATOZOID
DIAGRAM OF OVUM AND SPERMATOZOID
Embryology.—It is not necessary to describe here in detail the different changes which the two conjugated cells pass 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 testicles. 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).