KEIMBAHN-DETERMINANTS 233 



contents become visibly different in some animals, 

 and when the mature eggs develop normally these 

 "organ-forming substances" are segregated in def- 

 inite cleavage cells and finally become associated 

 with definite organs of the larva. 



Conklin (1905) 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 sub- 

 stances." 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 differentiated 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-forming substances rather than of 

 cleavage cells. The study of ctenophores, nemer- 

 tines, annelids, mollusks, 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 

 writer has also proved from his study on Phallusia 

 (1911) that these various substances exist even when 

 they are not visible in the living egg. It is interesting 

 also to note that Duesberg (1913) finds the "myo- 



