70 



THE FIRST CEREBRAL VESICLE. 



grey and white matter of the island of Reil becomes differentiated. The rest of the 

 wall of the hemisphere vesicle (mantle of Reichert), although remaining for a time 

 thinner than the floor, eventually thickens to form the whole of the grey and white 

 matter of the hemisphere, except along the line where the mesoblast, which is to 

 form the choroid plexus of the lateral ventricle, passes into the choroidal fold of the 

 neural epiblast, which becomes thinned out over the invading mesoblast and con- 

 verted into the epithelial covering of the plexus. 



The growth of the hemispheres takes place gradually. They extend at first some- 

 what forwards and upwards, separated by a thin layer of mesoblast which forms the 



Fig. 79. TRANSVERSE SECTIONS 

 THROUGH THE BRAIN OF A 

 SHEEP'S EMBRYO OP 27 CM. IN 

 LENGTH. (From Kolliker. ) 



In A the section passes through 

 the foramina of Monro, in B through 

 the third and lateral ventricles some- 

 what further back, st, corpus stria- 

 turn ; th, optic thalamus ; t, third 

 ventricle ; c, c', rudiment of internal 

 capsule and corona radiata ; /, lateral 

 ventricle with choroid plexus, pi ; 

 h, hippocampus major ; /, primitive 

 falx ; a, orbito-sphenoid ; sa, pre- 

 sphenoid : p, pharynx ; ch, chiasrna ; 

 o, optic nerve ; m, m, foramina of 

 Monro ; to, optic tract ; mk, Meckel's 

 caitilage. 



falx, but soon begin to pass 

 progressively backwards, so 

 that by the end of the third 

 month they have covered the 

 region of the optic thalami, by 

 the fourth month they have 

 reached the corpora quadri- 

 gemina, and by the sixth they 

 cover not only the corpora 

 quadrigemina, but also a great 

 part of the cerebellum, pro- 

 jecting even beyond this by 

 the end of the seventh month. 

 In front, and for some dis- 

 tance backward over the roof 

 of the third ventricle, where 

 the vesicles are not separated 

 by the falx, their mesial sur- 

 faces come into contact, and, 

 during the third month, partly 

 grow together, but in such a 

 manner as to leave anteriorly 

 just in front of the third 

 ventricle, a triangular area 

 free in the middle, but completely surrounded at its periphery by the united parts. 

 Thus is formed the cavity which is known as the fifth ventricle or ventricle of the 

 septum lucidum, which at no time has any communication with the vesicles of the 

 cerebral hemispheres, nor with any other of the cerebral vesicles. 



