398 EDWIN G. CONKLIN 



1. Cleavage of partial eggs. Driesch has said that the cleavage 

 of ^ blastomeres of Phallusia is neither whole nor half, but 're- 

 gellos-solid.' I also find that the cleavage of such eggs differs 

 from the whole or half cleavage of the normal egg in that the 

 blastomeres along the injured side turn in to a certain extent 

 over the injured surface thus causing the living half to become 

 spherical in form. However, the sequence, rate and differential 

 character of the divisions remains the same as in the half of a 

 normal egg, and the resulting blastomeres may still be identified 

 with those ot the normal cleavage. 



The cleavage of right or left I blastomeres is essentially like 

 that of f blastomeres, but differs in a typical manner from the 

 cleavage of anterior or posterior | blastomeres ; and the latter are 

 also different one from the other. All of the muscle-forming 

 substance is found in the two posterior quadrants of the egg. 

 The two small celfs which mark the posterior pole in most 

 ascidian eggs, and the rows of large muscle-forming cells which 

 run forward from these on both sides, clearly distinguish the pos- 

 terior f embryo from the anterior f one. In all of these regards 

 the cleavage of single blastomeres of Phallusia resembles that in 

 Cynthia. On the other hand it is undoubtedly much more diffi- 

 cult to identify the different quadrants of the egg, and the blas- 

 tomeres which are formed from them, in Phallusia, than in Cyn- 

 thia, and this is especially true in the later stages of cleavage. 

 On the whole, however, there is no reason to doubt that the 

 form of the cleavage of single blastomeres is the same in the two 

 genera, and that it differs from the normal, chiefly if not entirely, 

 in the fact that in most cases the surviving blastomere becomes 

 nearly spherical, thus causing the cells formed from it to close 

 in over the injured side. 



2. Gastrulation of partial eggs. There is no doubt that in 

 many cases the ^ or even the j egg of Phallusia may give rise to 

 a stage which closely resembles a small but typical cup-shaped 

 gastrula, but in every case which I have studied carefully this 

 resemblance has proved to be apparent rather than real. In 

 most cases it is due to the ingrowth of ectoderm cells over the 

 injured side, and in these cases the apparent mouth of the gastrula 



