Mosaic Development ui Ascidian Eggs. 199 



cleavage planes of the surviving half and the dead blastomere; 

 that after the third cleavage the cells occupy very different posi- 

 tions from the normal (Tetraeder, Halbtetraeder); that divisions 

 may be equal or unequal at the fourth cleavage, and finally that 

 the cleavage could not be regarded as partial ("halb") nor entire 

 ("ganz") but "regellos-solid." The evidence which Driesch 

 brings in support of this conclusion is of little value since it is plain 

 that he was unable to orient these cleavage forms and did not know 

 from what part of the original egg they came nor from what pole 

 they were viewed. My observations on the cleavage of isolated 

 uninjured blastomeres of the egg of Cynthia confirm and extend 

 the conclusions of Chabry and Crampton that the cleavage of such 

 blastomeres is unaltered save for slight changes in the direction 

 of some of the divisions; they are opposed to the conclusions of 

 Driesch that the cleavage of such blastomeres is inconstant and 

 irregular. 



2. Gastrula. 



Chabry figures four gastrul?e from isolated blastomeres, viz: 

 his Figs. 108, 114, 129 and 130. O. Hertwig, who copies Fig. 129 

 in his book, "Die Zelle" ('98), says that it is a normal typical 

 gastrula. Similarly Korschelt and Heider, who also copy this 

 figure in their text-book ('02), affirm that it is a normal small 

 gastrula. However, these authors bring no particle of evidence to 

 the support of this bare assertion; Chabry himself nowhere says 

 that any of the gastrulae figured by him are normal and the figures 

 themselves do not show that such is the case. On the other hand 

 I can positively affirm that a normal entire gastrula is never 

 formed from an isolated blastomere of the egg of Cynthia. In 

 the absence of any evidence in favor of Hertwig's and Korschelt 

 and Heider's interpretation and in the face of this positive evidence 

 against it I think it may safely be assumed that Chabry's figures 

 are not those of normal typical gastrulae. Crampton expressly 

 says that he did not carefully observe the process of gastrulation 

 in the embryos derived from isolated blastomeres of the Molgula 

 egg, but Driesch says that the process of gastrulation may be 

 easily observed in Phallusia, that a typical ascidian gastrula is 

 formed and that the closure of the blastopore takes place in the 

 normal manner. "Alles sind verkleinerte Aehnlichkeitsbilder der 

 Processe an normalen Eiern, welche stets vergleichen wurden." 



