416 



EMBRYOLOGY OF THE BRAIN AND SPINAL CORD. 



The roof-plate of the diencephalon stretches out and becomes 

 very thin, except at its posterior extremity (Figs. 118, V3, and 

 120). Its major portion forms the epithelium covering the third 

 ventricle. Posteriorly, fibers grow through it from opposite sides 

 and form a transverse white band, the posterior commissure. Im- 

 mediately in front of this commissure, a diverticulum of the roof- 

 plate appears which is the primitive pineal body. The pineal 

 body soon becomes solid and is joined to the diencephalon by a 



Fig. 124. Dorsal view of an embryonic brain, the roof of the lateral ventricles 

 having been cut away. Embryo of 12.6 mm. (McMurrich after His.) 



b. Brachium superius. eg. Corpus geniculatum laterale. Cp. Chorioid'plexus collateral 

 ventricle. Cqa. Colliculus superior of corpora quadrigemina. h. Hippocampus, hf. Hip- 

 pocampus and fissure. Ot. Thalamus. p. Pineal body. rp. Roof -plate. 



constricted stalk in which there is a conical prolongation of the 

 ventricular cavity, called the pineal recess (Fig. 120). Anterior 

 to the pineal body, two longitudinal folds of the roof-plate dip 

 down into the ventricular cavity. These are followed by two 



