480 'Journal of Comparative Neurology and Psychology. 



lamina terminalis. This is not very long in the embryo of 17 

 somites, but grows distinctly in length as the forebrain expands. 

 The apparent great thickness of the lamina terminalis in Fig. 3 is 

 of course due to the persistence of the fusion of ectoderm and brain 

 wall at this time. The primitive optic groove is deeper and- sharper 

 in outline than in the last stage. 



The relations in an embryo of 24 somites are shown in Fig. 16 

 representing the medial view of a model of the right half of the 

 head. In this is seen the depression of the primitive optic groove 

 until its wall comes into contact with the ectoderm and cuts off the 



r.n. 



Fig. 16. Squaliis ac, 24 somites, medial aspect of a model of the right half 

 of the head. X ''^^ 



front part of the preoral entoderm as before described. The lens- 

 shaped space in front of the primitive optic groove is filled chiefly 

 by mesectoderm from the terminal part of the neural crest. The 

 lamina terminalis is marked by a small triangular pit above, the 

 neuroporic recess, and by a rounded shallow pit below. This pit 

 occupies the lower portion of the neuropore space and may therefore 

 be called the terminal pit. The optic vesicle is now stalked and the 

 cavity of the stalk is seen near the base of the brain. It is notice- 

 able that this cavity no longer communicates directly with the primi- 

 tive optic groove. It seems equally closely related to the terminal 

 pit. Between the two pits the brain wall is somewhat thickened. 



