222 WILLIAM BATESON. 



behind the openings is figured (fig. 31). They here extend 

 along nearly the whole vertical depth of the archenteron. The 

 cells forming their outer wall are very granular, and project in 

 an irregular way into the cavity which they enclose. The 

 inner wall is formed of cells still in connection with the hypo- 

 blast, whose contour is also rounded and irregular. As yet 

 they only differ slightly from the rest of the hypoblast, being 

 smaller and somewhat more granular. The minute structure 

 of the layers in this region is shown in Fig. 31, a. The 

 cavities extend from the anterior groove to the posterior one, 

 — that is to say, throughout the length of the *' collar " at this 

 stage. 



Behind them the archenteric wall is simple and the body is 

 a two-walled elliptical cylinder, presenting nearly the same 

 appearances which were described as occurring immediately in 

 front of the collar. The archenteric walls are, however, some- 

 what thicker. This arrangement only occurs for a very short 

 distance, and is continued as far backwards as the thir d meso- 

 blastic region, which begins in front of the transverse band 

 and cilia. In the anterior end of this area the hypoblast is 

 split on each side, thus enclosing a pair of cavities similar in 

 appearance to those of the second mesoblastic region, but 

 differing from them in having a greater horizontal extent. The 

 cavity of the archenteron is therefore reduced in this part 

 of the body. These mesoblastic pouches open by large foramina 

 (fig. 34) into the lumen of the gut ; these openings occur in 

 the posterior third of their extent. As will be seen in fig. 33 

 the outer walls of these two cavities do not continue the curve 

 of the hypoblast, but spring from it, bending outwards, con- 

 sequently forming a pair of archenteric diverticula. 



In transverse section through the communicating foramina 

 the ventral wall of the archenteron is thicker than the dorsal. 

 Posteriorly this thickening increases, and, meeting the dorsal 

 wall, closes up the lumen of the archenteron. As previously 

 stated, the anus is not found until a later stage is reached. 

 This completes the account of the anatomy of the larva at this 

 period. 



