Facts and Factors of Development 13 



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centrosome, appears, either from the middle-piece or 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 cen- 

 trosome then approach the egg nucleus and ultimately the two 

 nuclei come to lie side by side (Fig. 4). Usually when one 

 spermatozoon has entered an egg all others are barred from en- 

 tering, probably by some change in the surface layer of the egg or 

 in the chemical substances given out by the egg. 



Oosperm or Zygote a Double Being. This union of a single 

 spermatozoon with an egg is known as fertilization. Whereas 

 egg cells are usually, but not invariably, incapable of development 

 unless fertilized, there begins, immediately after fertilization, a 

 long series of transformations and differentiations of the ferti- 

 lized egg which leads to the development of a complex animal, even 

 of a person. In the fusion of egg and sperm 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 of every cell, caused by the union of egg 

 and sperm, is never lost. 



A New and Distinct Individual. 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 practi- 

 cally 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 evident that the new individual comes 

 into being with the fertilization of the egg; rather the moment of 

 birth, or separation from the mother, is generally looked upon as 

 the beginning of the individual's existence. And yet in all cases 



