232 GERM-CELL CYCLE IN ANIMALS 



of the oogonia in Dytiscus a rosette of sixteen cells 

 is produced of which one is the oocyte and the other 

 fifteen nurse cells. The rosette thus formed possesses 

 a definite polarity coincident with the axis of the 

 oocyte which is identical with that which was present 

 in the last generation of oogonia. Similarly in 

 Miastor (Fig. 12) the polarity of the oocyte is recog- 

 nizable as soon as the mesodermal cells, which serve in 

 this species as nurse cells, become associated with it. 



The germ cells of other animals also possess a 

 precocious polarity, as evidenced by their implanta- 

 tion in the germinal epithelium (e.g., Wilson, 1903; 

 Zeleny, 1904, in Cerebratulus), the position of the 

 nucleus, the formation of the micropyle (Jenkinson, 

 1911), etc. This is true not only for the inverte- 

 brates, but, as Bartelmez (1912) claims, "the polar 

 axis persists unmodified from generation to genera- 

 tion in the vertebrates and is one of the fundamental 

 features of the organization of the protoplasm" (p. 

 310). Furthermore, experiments with centrifugal 

 force seem to prove that the chief axis of the egg is not 

 altered when substances are shifted about, but is 

 fixed at all stages (Lillie, 1909; Morgan, 1909; 

 Conklin, 1910). Bilaterality also is demonstrable 

 in the early stages of the germ cells of many animals, 

 and, like polarity, seems to be a fundamental charac- 

 teristic of the protoplasm. 



It is somewhat difficult to harmonize the various 

 results that have been obtained, especially by experi- 

 mental methods, from the study of egg organization. 

 As the oocytes grow, the apparently homogeneous 



