6 THE DEVELOPMENT OF 



differentiation of the entoderm forms the lower layer cells 

 until after the closure of the primitive groove, and that 

 then it separates from the mesoderm-entoderm, and later, 

 envelopes the whole yolk. The " yolk cells " form no part 

 of the permanent entoderm, but rather play the part of 

 vitellophags. 



BLASTOPORE AND ANUS. In staged, (fig. 10; section, 

 fig. 9) is shown the process of gastrulation. In describing 

 this stage (ante, p. 138) , I pointed out that the blastopore 

 was clearly behind the point where the abdomen was sub- 

 sequently to be formed, but said that I was unable to ascer- 

 tain whether any definite relationship existed between the 

 blastopore and the anus. A subsequent section, I regard 

 as throwing light upon this point. It is shown in fig. 29, 

 and passes in an obliquely longitudinal direction through 

 the inner edge of the optic lobes, the ventral bands, and 

 through the thoracico-abdominal area. In the latter re- 

 gion, it cuts through two pits, the anterior and larger being 

 the abdominal flexure (/"), while behind it is a second 

 and smaller pit, which certainly becomes the proctodeum 

 and which I now regard as being at the same time in the 

 position of the blastopore. In other words, the blastopore 

 occupies the same position as the anus and may be actually 

 identical with it. 



A comparison of this figure with Ishikawa's ('85) fig. 

 62 seems to lend countenance to the view that he has in- 

 terpreted the abdominal flexure as proctodeum. I regard 

 that depression in his figure, behind the letters " ab " as the 

 real anus and as homologous with the similar depression in 

 my figure cited. In support of this view, I would point out 

 that in both Astacus(Reichenbach, '86) andCrangon (vide 

 infra) , the anus is at first on the dorsal and only later at- 

 tains the position on the ventral surface, which it has in the 

 adult of all Crustacea. 



