552 



LOCY. 



[Vol. XI. 



plane, and in its median portion, the depression develops into 

 the infundibulum. 



The lateral depressions, which form the optic vesicle, sink 

 downwards more rapidly than the median part of the depression, 

 and as a result there is left a ridge or keel separating the optic 

 vesicle and connecting the anterior-median tip of the head- 

 plate with the central tongue-like process. Miss Piatt, in her 

 studies on the head of this animal, says : " It may be here 



K 



M> 





j:: 



^' 



J' -. ^v 



.-a«s^>sC 





^f 



-.ji^'-^yii-' 





Sk. 



6f 



Cut 8. — Eight transverse sections of the embryo shown in Fig. 8, PI. XXVI. x about 

 diameters. The numbers below the sections refer to their position in the series. 



3° 



noticed that the downward growth in the floor of the brain, 

 which gives rise to the infundibulum, is primitively bilateral, 

 and not median," and I think, therefore, that she saw these 

 circular optic pits in an early stage, but interpreted them as 

 lateral pockets of the infundibulum. There might be reason to 

 doubt whether they are the optic vesicles, if they could not be 

 traced with perfect continuity into the eyes, and if there were 

 not evidence of histological differentiation in their areas. 



By the time the neural folds of the head begin to rise the 

 optic vesicles are very prominent, and may be observed from 

 the outside and from within. Fig. 1 3 shows an embryo viewed 



