1 8 Journal of Comparative Neurology. 



tains the form and structure described up to the end of the 

 fourth day. In the beginning of the fifth day numerous large 

 blood vessels are developed over the roof of the mid-brain and 

 surround the pineal projection with a network. Toward the 

 middle of the fifth day small spherical diverticles grow out from 

 the wall of the projection into the surrounding connective tissues 

 and are forthwith constricted off as hollow spheres .02-. 03 mm. 

 in diameter. The constriction is brought about by means of 

 the surrounding connective tissue. The number of vesicles 

 constricted off at the end of the fifth day is limited, toward the 

 sixth and seventh day it increases and surrounds the pineal pro- 

 jection in the form of a wreath, consisting of the same elements 

 as the embryonic brain. The formation and cutting off of ves- 

 icles continues from the seventh to the twelfth days. Various 

 large spherules (from .03 to .05 mm. in diameter) and funnels 

 (.24 mm. long and .06 mm thick) surround the pineal process 

 (which is 1.5 mm. long), where the larger diverticles arise 

 through the later outgrowth. 



Thus over the roof of the thalamus a club-shaped struc- 

 ture is found which stretches far into the connective tissue and 

 is fastened to the skull roof by a thick connective tissue fibre at 

 its enlarged end. Under the pineal projection, in the mean- 

 while, the adjoining roof of the thalamus has been drawn out 

 to a funnel-formed appendage, which from its relation to the 

 pineal gland has been called the recessus infrapinealis. The an- 

 terior wall of the projection is formed from a part of the middle 

 vascular plexus, the posterior from the upheaved transverse 

 lamella of the post-cerebral commissure, and the right and left lat- 

 eral walls pass over into the optic lobe again. The cavity of 

 the pineal projection remains present to the 14th day of incuba- 

 tion, then it decreases to an insignificant size, and this at last is 

 changed to a vesicle. Up to the last day of incubation the ves- 

 icles retain their radial structure and connection of cylindrical 

 cells, then they proliferate at the periphery into a layer of round 

 cells, and the central cylindrical cells appear to have ciliated pro- 

 jections. The surrounding richly vascular connective tissue re- 

 mains between the vesicles as the stroma of the adult gland. 



