88 ALIMENTARY CANAL. 



slit-like aperture, wliich corresponds to the anus of Rusconi 

 (PL IV. fig. 7). 



The cavity anteriorly has a more or less definite form, 

 having lateral walls, as well as a roof and floor (PI. V. fig. 

 106 and 10c). Posteriorly it is not nearly so definitely en- 

 closed (PL V. fig. 10a). The ventral wall of the cavity is 

 formed by yolk. But even in stage B there are beginnings 

 of a cellular ventral wall derived from an ingrowth of cells from 

 the two sides. 



By stage C considerable progress has been made in the 

 formation of the alimentary canal. Posteriorly it is as flattened 

 and indefinite as during stage B (PL ix. figs. 26 and 2c). 

 But in the anterior part of the embryo the cavity becomes 

 much deeper and narrower, and a floor of cells begins to be 

 formed for it (PL ix. fig. 2); and, finally3 in front, it forms a 

 definite space completely closed in on all sides by cells (PL 

 IX. fig. 2a). Two distinct processes are concerned in effect- 

 ing these changes in the condition of the alimentary cavity. 

 One of these is a process of folding off the embryo from the 

 blastoderm. The other is a simple growth of cells inde- 

 pendent of any folding. To the first of these processes the 

 depth and narrowness of the alimentary cavity is due; the 

 second is concerned in forming its ventral walL The combi- 

 nation of the two processes produces the peculiar triangular 

 section which characterises the anterior closed end of the 

 alimentary cavity at this stage. The process of the folding off 

 of the embryo from the blastoderm resembles exactly the 

 similar process in the embryo bird. The fold by which the 

 constricting off of the embryo is effected is a perfectly con- 

 tinuous one, but may be conveniently spoken of as composed 

 of a head-fold and two lateral folds. 



Of far greater interest than the nature of these folds is the 

 formation of the ventral wall of the alimentary canal. This, as 

 has been said, is effected by a growth of cells from the two 

 sides to the middle line (PL [X. fig. 2). The cells for this 

 are however not derived from pre-existing hypoblast cells, but 

 are formed spontaneously around nuclei of the yolk. This fact 

 can be determined in a large number of sections, and is fairly 

 well shcAvn in PL ix. fig. 2?ia. The cells are formed in the 



