EvcLESHYNiER-DAVis, Epiphysis and Parapkysts in Amta. 4g 



the vesicle, the form of its lumen, and the relative thickness of 

 its walls are here further illustrated. The inner layer of the 

 superficial epiblast immediately above the vesicle, which in the 

 earlier stages is practically wanting, is now extending over this 

 area. A short distance in front of the vesicle there is a small 

 transverse fold of the brain wall ; this fold extends antero-ven- 

 trally and marks the boundary between the thalamencephalon 

 and prosencephalon. 



Larva 8-g days, j-6 mm. 



An examination of the living larva reveals no external pe- 

 culiarities which would indicate the position of the vesicle. 



A reconstruction of the vesicle at this stage closely resem- 

 bles Fig. 5, except that its stalk has narrowed, and there has 

 appeared a rounded protuberance in the proximal portion of its 

 dorsal wall. 



A median sagittal section is shown in Fig. 8. The contour 

 of the distal portion remains the same as in the previous stage. 

 In the dorsal wall the slight evagination referred to in the pre- 

 ceding stage is better defined, causing the lumen to widen at 

 this point. It is now apparent that we are here dealing with a 

 secondary vesicle {e2) formed as a diverticulum of the first. 

 Histologically its structure is essentially the same as the adja- 

 cent wall of the primary vesicle. 



Larva g days, 6-y mm. 



The living larva at this time shows pigment in the retina 

 and head region where it is distributed quite uniformly, except 

 in a mid-dorsal area which lies slightly anterior to the lateral 

 eyes. This area marks the position of the underlying primary 

 vesicle. 



A reconstruction of this stage ( Fig. 9 ) shows that the 

 primary vesicle is more sharply defined, owing to the fact that 

 its anterior margin has extended forward over its ventral por- 

 tion ; but aside from this it has undergone little change in out- 

 line. The reconstruction now plainly shows a well defined 

 smaller vesicle {e2) lying anterior to the primary and delimited 

 by a shallow groove. It lies, at first, so far as we have ob- 



