478 ROBERT W. HEGNER 



ing substances rather than of cleavage cells. The study of ctenophores, 

 nemertines, annelids, moUusks, ascidians and amphibians (the frog) 

 shows that the same is probably true of all these forms and it suggests 

 that the mosaic principle may apply to all animals (p. 221). 



The same wri'ter has also proved from his study on Phallusia 

 ('11) that these various substances exist even when they are not 

 visible in the living egg. It is interesting also to note that Dues- 

 berg ('13) finds the 'myoplasm' of Cynthia to be crowded with 

 plasmosomes, differing in this respect from other egg regions. 



Experiments, especially those of Lillie ('06), Morgan and 

 Spooner ('09), Morgan ('10) and Conklin ('10) have shown that 

 in many eggs the shifting of the supposed organ-forming sub- 

 stances has no influence upon development, and leads to the con- 

 clusion that these visible substances play no fundamental role 

 in differentiation, but that the invisible ground substance is 

 responsible for determinate development. The eggs of different 

 animals, however, differ both in time and degree of organization, 

 and the conflicting results may be accounted for by the fact that 

 specification is more precocious in some than in others. 



The most plausible conclusions from a consideration of these 

 observations and experiments are that every one of the eggs in 

 which Keimbahn-determinants have been described, consists 

 essentially of a fundamental ground substance which determines 

 the orientation ; that the time of appearance of Keimbahn-determin- 

 ants depends upon the precociousness of the egg; that the Keim- 

 bahn-determinants are the visible evidences of differentiation 

 in the cytoplasm; and that these differentiated portions of the 

 cytoplasm are definitely localized by cytoplasmic movements, 

 especially at about the time of maturation. 



B. The localization of the Keimhahn-determinants 



One of the characteristics of the Keimbahn-determinants is 

 their regular appearance at a certain stage in the germ cell cycle, 

 according to the species in which they occur, and their constant 

 localization in a definite part of the egg, or in one or more definite 

 cleavage cells. Keimbahn-determinants are recognizable in 



