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. 



51. Fertilization. The union of a sperm cell with an ovum 

 constitutes the act of fertilization. Often there is a special 

 aperture (micro pyle) 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 multiple fertiliza- 

 tion may occur. Multiple fertilization occurs normally in some 

 species. Such 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 centrosome and a very small 

 amount of cytoplasm. It at once organizes 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 num- 

 ber of chromosomes of the species, now draw together and organize 

 into a new nucleus. Thus is formed the first segmentation nucleus, 

 and the egg is fertilized. With the addition of the chromosomes 

 in the male nucleus the fertilized ovum contains the same 

 number of chromosomes as before maturation, which in each 

 species of animals is a constant number. It appears that 

 fertilization restores to the female cell essentially what it lost m in 

 the process of maturation, and in addition stimulates it to active 

 nuclear and cytoplasmic division as indicated in the next 

 paragraph. 



52. Segmentation or Cleavage. Following shortly upon 

 fertilization, if conditions are favorable, ordinary mitotic nuclear 

 division begins and the ovum divides promptly into 2, 4, 8, 16, 

 etc., cells (blastomeres). In these divisions the chromosomes 

 split, and one-half of each chromosome brought in by the sperm 

 nucleus and one-half of each furnished by the egg-nucleus go to 



