CASTLE. — CELL LINEAGE OF THE ASCIDIAN EGG. 211 



Postscript. 



Since the foregoing was written I have had the opportunity of mak- 

 ing further observations upon the Hviug egg of Ciona immediately 

 after fertilization and during the early stages of segmentation. I have 

 repeatedly seen the polar globules and observed continuously the 

 cleavaee staares following their formation. These observations lead to 

 the surprising but unavoidable conclusion, that the point on the sur- 

 face of the egg at which the polar globules form becomes later the 

 centre of the dorsal or endodermal half of the egg. The evidence for 

 this conclusion will be presented and its significance discussed in a 

 future paper. 



September 29. 1894. 



Postscript No. 2. 



After the preceding was already in print, Paul Samassa's recent 

 paper was received (Zur Kenntniss der Furchung bei den Ascidien. 

 Arch. f. mik. Anat., Bd. XLIV. Heft I. pp. 1-14, Taf. I. und II. 

 Ausgegeben 15 Sept., 1894). 



Samassa's conclusions agree in a ^ratifying manner with my own, 

 that Van Beneden et Julin on the one hand, and Seeliger on the other, 

 were mistaken in their orientation of the Ascidian egg. Here, how- 

 ever, the agreement ends, for my own interpretation of the cell lineage 

 differs radically from that of Samassa. 



In the earlier part of this paper I have pointed out how fatal to the 

 conclusions of Van Beneden et Julm was their conjecture as to the 

 manner of division in only three pairs of cells in the egg of Clavelina. 

 I cannot forbear .saying that a more apt illustration of the utter un- 

 trustworthiness of this method of determining cell lineage could not 

 have been offered than is found in Samassa's paper, where the method 

 of conjecture seems to have been followed in a wholesale and entirely 

 unjustifiable manner. 



In his account of the 32-cell stage Samassa makes D^-^ (his ell) the 

 sister cell of D®^ (his c 12) both in Ciona and in Clavelina. In doing 

 so he contradicts, though without giving a particle of evidence in sup- 

 port of his views, the recorded observations of Van Beneden et Julin 

 on Clavelina and of Chabry on Ascidiella, and this too in the face of 

 the fact that Van Beneden et .Julin offer the absolutely incontroverti- 

 ble evidence of karyokinetic figures in support of their interpretation. 



