THE PINEAL ORGAN OF MAMMALS 



303 



in man is a thickening of the ependyma in the posterior part of the roof 

 of the diencephalon and the adjacent parts of the alar laminae. This 

 thickening has the form of a longitudinal band which is slightly raised 

 above the general surface and is grooved on its under surface. It lies in 

 front of the posterior commissure, which commissure has already appeared 

 and extends backwards for a considerable distance above the ventricle 

 of the midbrain, which becomes the future aqueduct of Sylvius. Soon, 



RC. 



Fig. 208. — Median Longitudinal Section of a Rabbit Embryo (16 days, 

 11 mm.). The Apex of the Pineal Body is directed Forward, and the 

 Diverticulum shows a Central Constriction. (R. J. G.) 



Aq. C. : aqueductus cerebri. 



Cbl. : cerebellum. 



C.N. IV. : Cranial nerve IV. 



Inf. : infundibulum. 



P.C. : posterior commissure. 



P.O. : pineal organ. 

 R.P. : pouch of Rathke. 

 T. : tongue. 



V. III. : third ventricle. 

 V. IV. : fourth ventricle. 



by a forward growth of the anterior end of the thickened band and a 

 deepening of the groove, the rudiment assumes the typical form of the 

 primary diverticulum in lower classes of vertebrates : namely, a rounded 

 anterior segment with a small lumen which is separated by a slight con- 

 striction from the lumen of the basal segment, which opens freely into 

 the cavity of the third ventricle (Figs. 207, 208, 209.) 



In a 20-mm. human embryo the wall of the diverticulum shows a 

 division into the three typical zones of the neural tube, namely, an inner, 



