Development of the Mammal 287 



radiata, which consists of radially arranged 

 oval cells. 



The conditions just described are those of 

 the ovum after it is fully grown, but before 

 maturation has taken place. For the process 

 of oogenesis the reader is referred to any good 

 text-book of histology. 



FIG. 86. HUMAN SPERMATOZOA. (From Minot, 

 after Retzius.) 



A, complete spermatozoon. B, head seen from the side. 

 C ; extremity of the tail, e, end piece. //, head, m, main 

 piece. //, middle piece. All highly magnified. 



