46 Journal of Comparative Neurology. 



evagination inclines backward, later it grows in front until it in- 

 clines in this direction altogether. Its proximal portion always 

 hugs the roof of the brain, while the distal portion enlarges 

 upon all sides and takes the form of a hollowed vesicle placed 

 immediately under the epithelium. At the point of its junc- 

 ture with the brain the habenular ganglia are formed, that of 

 the right side, which is more developed from the beginning, 

 giving rise to the asymmetry of the superior wall of the brain 

 which is inclined toward the left side. The pineal vesicle then 

 lies lower on this side and deviates a little from the median line 

 at the commencement of the pineal evagination. This proba- 

 bly depends upon the habenular ganglia which begin to form 

 early, although it is not possible to infer from them the lateral 

 origin of the pineal. The narrow proximal part in the form of 

 a stalk at first unites the cavity of the pineal vesicle with that 

 of the brain. When the distal portion of the pineal vesicle in- 

 clines more in front, the stalk also inclines forward so that sev- 

 eral sections must be examined to follow its union with the 

 brain vesicle. The opening of the stalk narrows considerably 

 through the series. The opening of the pineal vesicle disap- 

 pears almost entirely and its superior wall becomes attached to 

 the inferior so that it is difficult to find the opening between 

 them. In the subsequent development the opening appears 

 again, the walls are separated and the vesicle alters its form. 

 The stalk of the organ also assumes another form. Its orifice 

 is obliterated and the stalk continues to lengthen, pushing the 

 terminal vesicle ahead in front. Immediately after its formation, 

 the pineal vesicle was behind the supra-commissure. In the 

 embryo of 6 mm. long it is above the habenular ganglia, later 

 it advances farther and farther beyond them. In a transverse 

 series of a larva 26 mm. long the stalk of the vesicle does not 

 lie at its posterior end, but a little in front. The part of the 

 stalk nearest the vesicle has a narrow opening which passes in 

 an oblique line from the nerve through the retina (the lower 

 wall of the organ) and enters the cavity of the organ about the 

 middle of the retina. This opening in traversing the retina is 

 only visible by the radiating group of cells which enclose it. 



