ORGAN-FORMING SUBSTANCES IN EGGS OF ASCIDIANS. 21 J 



forming substances ? The answer to both of these questions is 

 the same, viz., in the absence of a region or substance, the 

 organ to which it would normally give rise is not produced ; 

 and conversely each substance develops, if it develops at all, 

 into the parts which it would normally produce. Experiments 

 which I have carried out on ascidian eggs ' show that the de- 

 velopment of isolated blastomeres is strictly partial, as was first 

 shown by Chabry and afterwards denied by Driesch. As yet 

 I have been unable to get the isolated substances of the unseg- 

 mented egg to develop at all, but when they are isolated dur- 

 ing the cleavage stages they develop only into the parts which 

 they would normally produce, while the portions of the egg or 

 embryo which lack these substances develop into embryos which 

 lack the corresponding organs. Since the first cleavage of the 

 egg is bilaterally symmetrical and divides all the substances of 

 the egg equally, each of the first two blastomeres contains one 

 half of all the organ-forming regions and substances ; and since 

 isolated blastomeres of the ascidian egg always produce rounded 

 masses of cells which tend to close over the injured surface, 

 many of these half embryos have the appearance of whole em- 

 bryos of half size ; but a careful study of living material as well 

 as of stained preparations and sections shows that the larvae are 

 still incomplete up to the time of the metamorphosis. When the 

 division of the egg or embryo is made along any other plane 

 than the median one nothing even remotely resembling a normal 

 larva is obtained. Every substance of the egg develops, if it 

 develops at all, into the organs which it would normally pro- 

 duce, and while it has not been possible to isolate these sub- 

 stances in the unsegmented egg, their appearance is the same 

 before and after cleavage begins and under these circumstances 

 there is small room for doubting that even in the unsegmented 

 egg these are actually organ-forming substances. 



Therefore in the unsegmented egg and early cleavage stages 

 of Cynthia partita we have the most complete differentiation and 

 localization of the ooplasm ever yet reported for any egg. Apart 

 from the nuclei, the centrosomes and the asters, there are visible 

 in the 2-cell stage six different kinds of cytoplasmic substance, 



1 These experiments will be published in full elsewhere. 



