874 r,. DONCASTRR. 



appear to constitute a fold on each side {hd.). In sections 

 taken more anteriorly these two folds are continuous with 

 one another ventrally, and form a membrane covering the 

 anterior part of the mouth ; but further back they do not 

 reach so far down, and appear in section as flaps at the sides 

 of the head (figs. 23 — 26). There is a tendency for the two 

 layers of the hood to split apart from one another in the 

 region of its insertion, and so give rise to a cavitj^ [lid. cav.) 

 which is sometimes large in sections, but in life apparently 

 always narrow if present at all. 



It is also seen that the epidermis is thickened under the 

 hood, especially near the insertion of the latter; this con- 

 dition persists in the adult in the anterior part of the head, 

 and, since the thickening is in just the region from which the 

 hooks (^^ Greifhaken ") grow out, it is possible that it is 

 connected with their formation. Just at the corners of the 

 mouth there is an aggregation of nuclei which will give rise 

 to the lateral ganglia [gang. I.) (called vestibular b}' Grassi). 

 The nuclei plainly belong to the ectoderm, and some sections 

 show the rudiments of the ganglia extending further forward 

 than the mesoderm, and therefore Hertwig's supposition that 

 these ganglia belonged to the mesoderm must be regarded as 

 incorrect. On the dorsal surface a lens-shaped thickening of 

 the ectoderm is now visible on each side, forming the rudi- 

 ment of the eyes which appear in this stage (e.). 



The mouth and alimentary canal of the head have not 

 altered much on the third day, but the cell layers are 

 becoming more definite, and a small cavity has already 

 appeared. 



The mesoderm is beginning to be transformed chiefly into 

 muscle, but on the third day this change is not yet complete; 

 the cells, however, have a looser and less regular appearance 

 than before. 



During the fourth and fifth days the structures which were 

 outlined on the third day are further developed, and by the 

 sixth day they are approaching completion. Figs. 24 — 20 

 represent three sections througli the head of an individual of 



