114 



GENERAL EMBRYOLOGY 



next entire chapter. In the history of the spermatozoa a fourth 

 period is to be distinguished, namely, the period of transforma- 

 tion or metamorphosis; for the highly differentiated structure of 

 the spermatozoon is rapidly assumed after the process of 

 maturation is completed. This period is not marked in the 

 history of the ovum, for this, with the exception of its unusual 

 size, is not so markedly differentiated in structure. 



During the first of these periods, that of multiplication, the 

 cells of the reproductive tissue are termed oogonia and sper- 



Primordial Germ 



Cell ("Primitive 



Ovum"). 



Period of Multiplica- 

 tion . c h r o m o- 

 somes. (The num- 

 ber of cell genera- 

 tions is much 

 greater than indi- 

 cated here.) 



Period of Growth, 

 a chromosomes. 



Period of Matura- 



tion . 



c h r o m o- 



Oogonia. 



Primary Oocyte. 



Secondary Oocyte 

 and First Polar Body. 



Mature Ovum and 

 Three Polar Bodies. 



FIG. 60. Diagram of the chief events of 06 genesis. Adapted from Boveri. 

 Compare with Fig. 61. 



and of these there may be a great many generations 

 during this period, before growth commences. As the oogonia 

 and spermatogonia become older, division becomes slower and 

 ceases as the cells enter upon their growth period. At the close 

 of the growth period, while still contained within the ovary or 

 testis, the cells are known respectively as the primary ooeytes, 

 or ovarian eggs, and the primary spermatocytes. From this 

 point onward the histories of the eggs and sperm are not quite 

 identical, although entirely equivalent (Platner, O. Hertwig). 



