EAELY STAGES IN DEVELOPMENT OF BALANOGLOSSUS. 219 



the egg, and in the absence of a mouth, it is tolerably certain 

 that this growth must be due to the taking in of water con- 

 comitantly with the using up of the yolk particles, deposited 

 especially in the hypoblastic tissues. 



An attempt is made to indicate this constitution of the cells 

 where it occurs by a blurred shading in the figures. 



The nuclei of these cells, and of all the cells of this layer, are 

 irregular in shape. 



The cavity, lined by these cells, which will be spoken of as 

 the anterior body cavity (fig. 26, be, \), still communicates 

 with the original cavity of the archenteron, and the layer of 

 cells which forms its inner wall is still directly continuous with 

 the hypoblast itself. This continuity is shown in fig. 27. 



The gut here projects into the anterior body cavity as a tube 

 the end of which is obliquely truncated (fig. 35), so that the 

 ventral lip projects further forwards than the dorsal. In a 

 section taken behind that shown in fig. 27 the archenteron is 

 therefore seen as an elliptical structure, lying inside the anterior 

 body cavity, complete on its dorsal side. 



The free edges of the hypoblastic tube are continuous wuth 

 the body cavity epithelium, which is reflected backwards from 

 it, owing to the backward prolongation of the cavity itself. 

 This backward prolongation of the anterior body covity is not 

 of the same extent on all sides. Vcutrally to the gut, it is very 

 slight, and occurs in very few sections. It appears in fig. 27 

 as a small space below the ventral wall of the gut. On the 

 dorsal, and especially on the lateral aspects of the body, the 

 posterior parts of the anterior mesoblast are more conspicuous, 

 and will be subsequently shown to be of considerable 

 importance. 



Now, since the anterior body cavity is continued behind the 

 end of the gut on all sides excepting the ventral, it is crescentic 

 in shape, the concavity being directed downwards. This ap- 

 pearance exists only for a short distance. Behind it the con- 

 tinuity across the dorsal surface ceases, and the mesoblast 

 exists as a pair of small, hollow cavities at the dorso-lateral 

 sides of the gut, which is here much more fully developed, 



