4$ Journal of Comparative Neurology^ 



is not the case in the proximal portion of the dorsal wall ; here 

 is an area {e2) which remains thickened and is destined to play 

 an important part in the later history of the vesicle. The ven- 

 tral wall of the vesicle is from three to four layers of cells in 

 thickness. It is noticeable that all the cells of the vesicle pre- 

 sent, now and in subsequent stages, a somewhat different ap- 

 pearance from those forming the roof of the brain, in that they 

 are less deeply stained, larger, and approach a spherical form. 



A transverse section (Fig. 4) along the line 44 of Fig. 3 

 shows the dorso-ventral compression of the primary vesicle, 

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

 its walls. 



The epithelium {t'P) above the vesicle is but a single layer 

 of cells, while around this area it thickens to a double layer as 

 shown in the figure. Between the epithelium and the brain 

 scattered mesoblastic cells are always present, except in the 

 area just above the vesicle.' 



Latva S-y days, ^ 5 mm. 



A reconstruction of the primary vesicle in a larva at this 

 stage is shown in Fig. 5. Viewed from above it presents pos- 

 teriorly a more or less oval appearance, while anteriorly it grad- 

 ually narrows as it passes over into the proximal portion which 

 connects it with the brain wall. 



A sagittal section (Fig. 6) taken along the line 6-6 of Fig, 

 5 shows that the vesicle has changed but little in general out- 

 line. The dorsal wall throughout the greater portion shows no 

 changes beyond those described in the preceding stage. In the 

 region of the proximal thickening {e2) there is an increased 

 proliferation of cells. In this vicinity we can, for the first time, 

 distinguish a slight evagination of the wall of the primary vesicle. 

 It is important to note just here that this evagination cannot be 

 considered as an evagination of the brain wall, but is far up on 

 the dorsal wall of the primary vesicle. Its ventral wall shows 

 no marked change. 



A transverse section is shown in Fig. 7, its position being 

 indicated by the lines 7-7 of Fig. 6. The general outline of 



