230 



REPRODUCTION 



growth that occurs in the female germ cell before the maturation 

 divisions. By this means the greater size of the mature ovum is 

 attained. In many animals only two polar bodies are found upon 

 the surface of the egg (Fig. 211, p. 402). Examination shows 



ac& 



Fig. 115. — The spermatozoon. 



A and B, human spermatozoon, two views showing flattening of the "head" (nucleus) 

 region. C, stage in formation of spermatozoon of guinea-pig, showing what is more obvi- 

 ously a cell. D, diagrammatic figure of fully formed spermatozoon of a guinea-pig. aca, 

 acrosome; a./., axial filament; cy, cytoplasm; fi, flagellum; n, nucleus; to p., middle 

 piece. (A and B after Retzius; C and D after Meves. Reproduced from figure in Curtis, 

 "Science and Human Affairs," by permission of Harcourt, Brace and Co.) 



that these are cases in which the first polar body fails to divide, 

 as shown by the schematic representation (Fig. 114). The 

 time of polar-body formation also varies. In some animals 

 both maturation divisions occur before the egg leaves the ovary 

 and well in advance of fertilization; in others the first maturation 

 division occurs when the egg is mature and the second only when 

 the egg is stimulated by the entrance of a spermatozoon in fertiliza- 

 tion. In still other cases, both maturation divisions are delayed 

 until the time of fertilization. These differences in the time of 

 maturation in no wise affect the fundamental nuclear behavior 

 involved. 



As a result of the nuclear changes in oogenesis and spermato- 

 genesis, the mature ovum and spermatozoon each contain one-half 

 the number of chromosomes characteristic of all other cells in the 



