220 Edivin G. Conklin. 



and neural plate cells in the other a notochord, sense vesicle, or 

 tail is never formed and nothing resembling a normal larva 

 develops from either half. The anterior half never contains 

 muscle cells; the posterior half contains many muscle and mesen- 

 chyme cells, but evidently no chorda or neural plate cells. 



III. Conclusions, 



i6. My results confirm and extend those of. Chabry, but they 

 are fundamentally unlike those of Driesch; I agree with the work 

 of Crampton as to the cleavage of isolated blastomeres but cannot 

 agree with him that whole embryos or larvae are ever formed from 

 isolated blastomeres of the ascidian egg. 



17. Regulation in the ascidian egg and embryo is limited to 

 the closing of the embryo and the consequent extension of certain 

 cells from the uninjured to the injured side; and also to the forma- 

 tion of a typical notochord and an atypical sense vesicle in right 

 or left half embryos. One ooplasmic substance never gives rise 

 to another nor does a given type of cell ever produce cells of another 

 type or organs of a different kind than those which would arise 

 from it in a normal embryo. The fact that the power of regulation 

 is apparently greater in the adult ascidian than in the egg or 

 embryo may be deceptive; the injury to the egg which wipes put 

 completely certain ooplasmic substances may be really greater 

 than any which may be repaired in the adult. Furthermore it is 

 possible that the very rapid development of ascidians may act 

 as a check on regulation. 



18. These results prove that at least five of the substances 

 which are present in the egg at the close of the first cleavage, viz: 

 ectoplasm, endoplasm, myoplasm, chymoplasm, and chorda- 

 neuroplasm, are organ-forming substances. 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. Although I 

 have been unable to test the potencies of these substances before 

 cleavage begins, there seems to be no reason for supposing that 

 they are ever totipotent. Three of these substances are clearly 

 distinguishable in the ovarian egg and I do not doubt that even at 

 this stage thev are differentiated for particular ends. 



