STUDIES ON GEEM CELLS 477 



polarity of the oocyte is recognizable 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 implantation in the germinal 

 epithelium (e.g., Wilson, '03; Zeleny, '04, in Cerebratulus), the 

 position of the nucleus, the formation of the micropyle (Jenkinson, 

 '11), etc. This is true not only for the invertebrates, but, as 

 Bartelmez ('12) claims, "the polar axis persists unmodified from 

 generation to generation in the vertebrates and is one of the 

 fundamental features of the organization of the protoplasm" 

 (p. 310). Furthermore, experiments with centrifuged 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, '09; 

 Morgan, '09; Conklin, '10). Bilaterality also is demonstrable 

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

 polarity, seems to be a fundamental characteristic of the pro- 

 toplasm. 



It is somewhat difficult to harmonize the various results ob- 

 tained, especially by experimental methods, from the study of 

 egg organization. As the oocytes grow, the apparently homo- 

 geneous contents become visibly different in some animals, and 

 when the mature eggs develop normally these 'organ-forming 

 substances' are segregated in definite cleavage cells and finally 

 become associated with definite organs of the larva. 



Conklin ('05) has shown "that at least five of the substances 

 which are present in the egg [of Cynthia] at the close of the first 

 cleavage, viz., ectoplasm, endoplasm, myoplasm, chymoplasm, 

 and chordaneuroplasm, are organ-forming substances." Under 

 experimental conditions 



. . . . they develop, if they develop at all, into the organs which 

 they would normally produce; and conversely, embryos which lack 

 these substances, lack also the organs which would form from them. 



. . . . Three of these substances are clearly distinguishable in the 

 ovarian egg and I do not doubt that even at this stage they are differ- 

 entiated for particular ends (p. 220) The development of 



ascidians is a mosaic work because there are definitely localized organ- 

 forming substances in the egg; in fact the mosaic is one of organ-form- 



