52 Journal of Comparative Neurology. 



Latva 14.-J5 days, p-io mm. 



The living larva from this time on usually shows, externally, 

 no trace of the translucent area which in an earlier stage marks 

 the position of the underlying vesicles. In some cases we have 

 noticed it still present in the 11-12 mm. larva; its disappear- 

 ance is due, as will be seen, to the formation of pigment be- 

 neath this area. 



A reconstruction of this stage is shown in Fig. 20, The 

 primary vesicle when viewed from above is much the same in 

 general outline as in the previous stage, but when seen from 

 the side its dorsal growth is apparent. The secondary vesicle 

 has likewise grown, especially in length, and has shifted its po- 

 sition to the left and downward. Its long axis now coincides 

 with the long axis of the body. 



A sagittal section slightly magnified (Fig. 21) shows the 

 relation of the epiphysis to the other parts of the brain. Fig. 

 22 represents a more highly magnified view of the same sec- 

 tion through the vesicles. The primary vesicle is more or less 

 oval in outline, tapering to a point at its distal end. The stalk is 

 narrow at its base, but broadens as it merges into the distal por- 

 tion. It possesses a compressed, but well defined, lumen which 

 ventrally opens into the third ventricle ; dorsally it forms the 

 cavity of the vesicle. This cavity is bounded above by a wall 

 two rows of cells in thickness, while the ventral wall is several 

 rows of cells deep. It will be seen in this section that pigment 

 has formed in the region immediately above the vesicles. At 

 this time the superior commissure (sc.) is differentiated and 

 lies in close contact with the stalk of the primary vesicle. 



Figs. 23, 24, 25, 26 represent transverse sections along 

 the lines indicated by these numerals in Fig. 20. 



Fig. 24 shows a longitudinal section of the superior com- 

 missure. We have carefully studied the relation of this com- 

 missure to both vesicles, but have been unable at this stage to 

 trace fibers into either. Fig. 25 illustrates the further differ- 

 entiation of stalk and vesicle. It also shows the overgrowth of 

 the primary over the secondary vesicle. It is noteworthy that 



