tozoa as well as the ova of each individual differ slightly from those of every other individual.
2. Fertilization.—If a spermatozoon in its swimming comes into contact with a ripe but unfertilized egg, the head and middle-piece of the sperm sink into the egg while the tail is usually broken off and left outside. The nucleus in the head of the sperm then begins to absorb material from the egg and to grow in size and at the same time a minute granule, the centrosome, appears, either from the middle-piece or
Fig. 2. Two Human Spermatozoa. A, showing the surface of the flattened head; B, its edge; H, head; M, middle piece; T, tail. (After G. Retzius.)
from the head of the sperm, and radiating lines run out from the centrosome into the substance of the egg. The sperm nucleus and centrosome then approach the egg nucleus and ultimately the two nuclei come to lie side by side. Usually when one spermatozoon has entered an egg all others are barred from entering, probably by some change in the chemical substances given out by the egg.
This union of a single spermatozoon with an egg is known as fertilization. Whereas egg cells are usually, but not invariably, incapable of development without fertilization, there begins, immediately after fertilization, a long series of transformations and differentiations of the fertilized egg which leads to the development of a complex animal—of a person. In the fusion of the egg and sperm cells a new individual, the oosperm, comes into being. The oosperm, formed by the union of the two sex cells, is really a double cell, since parts of the egg and sperm never lose their identity, and the individual which develops from this oosperm is a double being; even in the adult man this double nature, caused by the union of egg and sperm, is never lost.
In by far the larger number of animal species the oosperm, either just before or shortly after fertilization, is set free to begin its own individual existence, and in such cases it is perfectly clear that the fertilization of the egg marks the beginning of the new individual. But in practically every class of animals there are some species in which the fertilized egg is retained within the body of the mother for a varying period during which development is proceeding. In such cases it is not quite so evident that the new individual comes into being with the fertilization of the egg—rather the moment of birth or the separation from the mother is generally looked upon as the beginning of the individual existence. And yet in all cases the egg or embryo is always distinguish-
