212 PROCEEDINGS OF THE AMERICAN ACADEMY. 



In the living egg of Ciona I have myself repeatedly seen the cell di- 

 visions leading to the 32-cell stage take place, and my observations 

 agree perfectly with those of Van Beneden et Julin on Clavelina, and 

 of Chabry on Ascidiella, viz. that the cell D®-^ (Samassa's ell) is the 

 sister cell of D''-^ (Samassa's c 21) and D^-^ (c 12) that of D«* (c 22). 

 If Samassa expects his statement to stand against all these independent 

 observations, he must present some evidence in its support. 



After discussing the 48-cell stage of Ciona, Samassa says : " Die 

 vi^eiteren Theilungen habe ich nicht Zelle filr Zelle verfolgt, da dies 

 fiir die Losung der mich interessirinder Frage nicht von Bedeutung 

 ist." Yet, again without a particle of evidence and confessedly without 

 detailed observations, he calmly proceeds to declare the ancestry and 

 fate of the individual blastomeres of subsequent stages ! The history 

 cell for cell of the cleavage subsequent to the 48-cell stage is precisely 

 the critical point in deciding the question with which Samassa is con- 

 cerned, viz. the origin of the endoderm, and in stopping where he 

 did he has stopped short of solving his problem. Worse than this, 

 by his guesses at the cell lineage he has fallen into positive error. 

 For example, the cells 8 and 9 of his Figure 10, wh'ich he says are 

 ectoderm, are not so at all, but are mesoderm. They with their mates 

 in the left half of the egg ultimately form the greater part, perhaps all, 

 of the lonoitudinal musculature of the tail, being derived from cells 

 corresponding to D^-^'^ and d®*^ of my Figure 12. There is no mis- 

 taking the identity of these cells , the large size of the nucleus, which 

 Samassa has observed for his cell 8, and would have found to hold 

 good at a slightly different stage for 9 also, is one of their distinguish 

 ing characteristics; their peculiar stainability under proper treatment is 

 another. Samassa assures us that these cells are derived from the 

 ventral or ectodermal cells of the 8-cell stage, and represent, with a 

 few cells posterior to them, " die erste Anlage der Medullarwiilste." 

 With these statements I squarely take issue. Of those under con- 

 sideration, only the cell 8 (my D^-^*') with its mate in the left half 

 of the egg, is derived from the ventral cells of the 8cell stage, the 

 other cell (9) is derived from the dorsal cells of the 8-cell stage ; and 

 both 8 and 9 form, not ectoderm, but mesoderm, beincj invasinated with 

 the endoderm cells at gastrulation. 



In view of these facts, and others which need not be presented at 

 this time, I am forced to take issue with Samassa's main conclusion, 

 "dass bei Ciona und Clavelina durch die dritte Furchung die Tren- 

 nung der beiden primiiren Keimbljitter erfolgt." For Ciona at least 

 this is not true, and the close correspondence which Samassa himself 



