402 EDWIN G. CONKLIN 



in the embryos of Phallusia, in spite of the fact that regulation is 

 in general more complete than in Cynthia. 



It follows from what has gone before that | larvae are even 

 more incomplete than ^ or | ones. The | blastomeres never 

 give rise to tissues or organs which they would not have formed 

 as parts of a typical larva; an anterior I produces a portion of 

 the neural plate, eye spot and various chorda cells, but no muscle 

 cells (fig. 11) ; a posterior | gives rise to muscle cells but no neu- 

 ral plate or chorda cells (fig. 12). Furthermore no I larva has 

 the external form of a typical larva with 'body' and tail, but 

 is only a rounded mass of cells (figs. 11, 12). 



CONCLUSION 



The egg of Phallusia mamillata, although it appears perfectly 

 homogeneous and as clear as glass in the living condition, shows 

 when fixed and stained, the same fundamental differentiations 

 of the ooplasm as have been found in the egg of other ascidians. 

 Just as in the cases of Ciona and Cynthia, a cap of protoplasm 

 which stains deeply with eosin gathers at the vegetative pole 

 immediately after the entrance of the spermatozoon into the egg, 

 and subsequently forms a crescent around the posterior half of 

 the egg. This crescent ultimately gives rise to the muscle and 

 mesenchyme cells of the tail of the larva, and is thus homologous 

 with the mesodermal crescent of Cynthia. The substances 

 which go into the ectoderm and endoderm cells of Phallusia are 

 histologically distinguishable and are localized in the 2-cell stage 

 exactly as they are in Cynthia and Ciona. The distribution of 

 these ooplasmic substances to the cleavage cells, the form of the 

 cleavage and the cell-lineage of the principal org,ans of the larva 

 is apparently precisely the same in Phallusia, Cynthia and Ciona. 

 In the organization and early development of the egg there is 

 no fundamental difference between Phallusia and other ascidians. 



A study of the development of ^, | and I blastomeres of 

 Phallusia mamillata shows that in this species, as in Ascidia 

 aspersa, Cynthia partita, Ciona intestinalis and Molgula manhat- 

 tensis, entire larvae do not develo'p from single blastomeres of 



