IG. 8. Attraction of chromosomes by centrosomes.
FIG. 9. Concentration of nuclei. Division of cell.
FIG. 10. Formation of new chromatin.]

[Illustration: 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 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).

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 tu