124 . THE DEVELOPMENT OF 



labelled "en" (endoderm), and the dorsal organ " gast. 

 mo." (gastrula mouth). It would seem as if the only in- 

 terpretation to be placed on these facts is that the serosa 

 is regarded as the endoderm and the dorsal organ as the 

 gastrula mouth. In a note on p. 261, Dr. Ayers modifies 

 some of these statements in the light of Balfour's re- 

 searches on the embryology of Peripatus. He now re- 

 gards the primitive groove as produced by an elongation 

 of the blastopore and says that in some insects this groove, 

 with the mesoderm arising from it, is to be regarded as 

 the only indication of the previous existence of a gastrula 

 mouth. This of course modifies many of the other con- 

 clusions summarized above, but to what extent does not 

 readily appear from the text. 



Patten says ('84) that in the Phryganids all the nuclei 

 of segmentation migrate to the surface and take part in 

 the formation of the blastoderm, leaving the yolk entirely 

 free. Then (p. 573) " the endoderm arises from any 

 point in the blastoderm by delamination, and the process 

 may continue even after the blastoderm has been converted 

 into the ventral plate." In support of this view he figures 

 (pi. XXXVI B, fig. 5) a section of an egg with the ven- 

 tral plate well-differentiated, in which cells which he re- 

 gards as yolk cells or endoderm, are budding from the 

 dorsal portion of the blastoderm (serosa). In another 

 place (p. 572) he says that the cells arising from the prim- 

 itive groove ("gastrula") are to be regarded as both 

 mesoderm and endoderm, and farther on he describes 

 and figures amoeboid cells, like those mentioned above, 

 budding from the mesoderm and extending into the yolk. 

 What the fate of these latter is he is not prepared to say, 

 but he is not ready to affirm that the result of this is to 

 increase the number of yolk cells. We may note in 

 passing, that Doctor Patten mentions the fact that no 



