lyO Ediuin G. Coiiklin. 



quite evident that not one of them resembled in any respect what- 

 ever a normal larva. In some cases both halves survived, as 

 shown in Figs. 77-81, in other cases one half only survived. In 

 all cases the surviving halves became rounded in form after the 

 operation, the more seriously injured cells being crowded out of 

 the embryo and forming a cellular mass of debris within the cho- 

 rion. In every instance the surviving halves remained within the 

 chorion, which was sometimes infolded as shown in Figs. 77-80. 

 Each half was surrounded by a layer of clear ectoderm cells; the 

 yellow cells were always found exclusively in the posterior half, 

 the gray endoderm cells largely in the anterior half. Nothing 

 resembling a notochord or neural tube ever developed in either 

 half and no structure resembling a tail was ever formed. In fact 

 these half embryos produced by cutting the early gastrulae in two 

 were altogether like the anterior and posterior half embryos which 

 I have already described, {cf. Figs. 77-82 and Figs. 47-58.) 



These results were so definite and conclusive that I did not 

 continue the experiments and I regret now that I did not also cut 

 gastrulae in two along the median plane, though there is no reason 

 to doubt that the results would be the same as in cases where one 

 of the first two blastomeres is killed. 



Comparing these results with those of Driesch, only one of two 

 explanations is possible. Either Phallusia must differ most 

 fundamentally from Cynthia, or Driesch must have mistaken the 

 median for the transverse plane in these cup-shaped gastrulae. 

 That the former possibility is not probable is evidenced bv the 

 fact that the cell-lineage of all ascidians so far studied is essen- 

 tially the same; furthermore my results as to the development of 

 anterior and posterior halves of the egg of Cynthia are confirmed 

 by my experiments on Molgula, as well as by Chabry's experi- 

 ments on Ascidia aspersa. There is every reason to believe that 

 what is true of these three genera is also true of Phallusia. On the 

 other hand there are certain evidences that Driesch may have 

 mistaken the transverse plane for the median; on p. 56 he says, 

 " Aber auch an der Bechergastrula kann man die ki'inftige Mediane 

 und also auch die Hauptrichtungen senkrecht zu ihr unterschieden: 

 es verlaufen namlich die Zelltheilungsgrenzen des Ektoderms 

 dieser Objecte so, dass sie gerade in der Medianen eine iiber die 

 ganze Oberfliiche fortgesetzte, nur sehr wenig gebrochene Ein- 

 heitslinie bilden (S. z. B. Castle, Fig. 62, 71) welche ohne Weiteres 



