36 ZOOLOGY 



mature well-nourished egg as the result of the two divisions. In 

 the formation of the sperm there are usually four perfect cells 

 arising from each spermatocyte. 



In the above discussion no account is taken of the fact that 

 the male and the female of a species may differ in the number 

 of chromosomes found both in body cells and in germ cells. 

 Frequently, for example, the oogonia have an even number {2x) 

 and the spermatogonia one fewer, an odd number (2% — i). 

 In such cases all the eggs will receive the same number of chromo- 

 somes (x). The odd, unpaired chromosome in the spermato- 

 cyte does not split, but goes undivided to one pole. This 

 results in two kinds of spermatozoa, equal in numbers, one 

 having x chromosomes and the other with x — i. When these 

 sperm cells unite with ova there arise two kinds of individuals, 

 one with 2x chromosomes in the body cells and germ cells, and 

 the other having 2a; — i. This is the condition that obtained 

 in the original female and male parents. 



SI. Fertilization. — The union of a sperm cell with an ovum 

 constitutes the act of fertilization. Often there is one (or more) 

 special aperture (micropyle) in the outer egg-membrane through 

 which the spermatozoon finds entrance. Usually only one 

 sperm cell gains admission to the interior of the ovum, whether 

 by way of the micropyle or through the unmodified membrane. 

 Changes normally occur in the membrane as soon as one sperm 

 enters, by which all others are excluded. In eggs which have 

 been kept too long or subjected to unfavorable conditions, the 

 response of the membrane may not be so quickly effected and 

 several spermatozoa may enter. Such polyspermy occurs nor- 

 mally in some species. Multiple fertilizations may produce 

 monstrosities. The sperm cell may enter the egg even before 

 the polar bodies are formed; or it may enter after maturation 

 is completed. It brings into the egg the nucleus, the centro- 

 some and a very small amount of cytoplasm. It at once organ- 

 izes itself as a second nucleus of the egg and is nourished by its 

 substance. The sperm nucleus and the egg nucleus, each 

 carrying one-half the full number of chromosomes of the species, 

 now typically draw together and organize into a new nucleus. 



