526 TEE POPULAR SCIENCE MONTHLY 



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 



. .T... M H 



«o» 



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 centro- 

 some 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 chemi- 

 cal substances given out by the egg. 



This union of a single spermatozoon with an egg is known as fertili- 

 zation. Whereas egg cells are usually, but not invariably, incapable of 

 development without fertilization, there begins, immediately after fer- 

 tilization, 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 fer- 

 tilization 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- 



