128 George Lejevre 



observed in the living embryo as well as followed from an exam- 

 ination of sections. It seems to be perfectly normal in charac- 

 ter in a great many parthenogenetic embryos, although I have 

 not been able to compare the origin of the entoblastic plate with 

 its normal cell lineage. Sections, however, show a close corre- 

 spondence with Torrey's description. The lower poleof theblas- 

 tula flattens, while the upper becomes somewhat arched, and the 

 entoblastic cells elongate and sink bodily into the cleavage cavity 

 which soon becomes nearly filled with them. Earlystagesof gas- 

 trulation are illustrated by the sections drawn in Figs. 40 and 41, 

 which closely correspond with Torrey's Fig. 10 (p. 232). After 

 sinking in, the entomeres multiply rapidly and form a rounded 

 mass which withdraws a little from the body wall. The enteron 

 now consists of a thick epithelium surrounding a small lenticular 

 cavity which later becomes greatly enlarged, while the cells com- 

 posing its walls are densely filled with yolk spheres. Fig. 42 

 shows the entoblastic mass with its early cavity, and, although the 

 •section does not pass through the blastopore, it is very similar to 

 Torrey's Fig. 6 A (p. 2^07). 



The blastopore, an elongated slit (Fig. 9), shifts from the lower 

 pole, where it first appears, to the future ventral side, until it comes 

 to lie close under the prototroch. Although I have not followed 

 its history in all particulars, the changes which it undergoes in the 

 parthenogenetic embryos evidently agree with the account given by 

 Torrey, who describes the enlargement of the anterior end of the 

 slit-like opening and the persistence of this portion to form the 

 mouth, while the posterior portion closes by approximation of its 

 sides. 



After gastrulation has taken place, an invagination of the ecto- 

 dermal cells around the blastopore occurs and gives rise to the oesoph- 

 agus (Fig. 43), the blind end of which abuts against the closed 

 enteron. In the formation of the oesophageal invagination, three 

 large cells, the oesophagoblasts, are distinguished from the rest by 

 their size and come to occupy a definite position in the wall of the 

 oesophagus, as described by Torrey (p. 205). The continuous 

 history of these cells has not been followed in my material, but 

 that the same specialized cells are present in the parthenogenetic 



