440 



EMBRYOLOGY. 



convex. Where the plane and convex surfaces are continuous with 

 each other there is a sharp bend in the mantle (Mantelkante). 



The vesicles of the hemispheres at first have 

 thin walls formed of several layers of spindle- 

 shaped cells (fig. 251, i) and each encloses a 

 large cavity, the lateral ventricle (fig. 251), 

 which is derived from the central canal of the 

 neural tube. Inasmuch as these have been 

 reckoned by the earlier authors as the first and 

 second ventricles, it is plain why the cavities 

 of the between-brain and medulla oblongata are 

 respectively designated as the third and fourth 

 ventricles. In Man, during the earlier months, 

 each lateral ventricle is in communication with 

 the third ventricle by means of a wide opening, 

 the primitive foramen of MONRO (figs. 239 ML 

 and 254 m). 



Tig. 250. -Brain of a 

 human embryo seven 

 weeks old, parietal 

 (Scheitel) aspect, after 



MlHALKOVICS. 



mgp, Interpallial (longi- 

 tudinal) fissure, at the 

 bottom of which is seen 

 the embryonic lamina 

 terminalis (Schluss- 

 platte) ; hms, left hemi- 

 sphere ; zh, between- 

 brain ; ink, mid-brain ; 

 Mi, hind - brain and 

 after-brain. 



Anterior to the foramen of MONEO lies the part of 

 the wall of the cerebrum which was infolded by the 

 development of the great interpallial fissure : on the 

 one hand it effects the anterior union of the walls of 

 the two hemispheres ; on the other it bounds the third 



ventricle in front, and is therefore called the anterior closing plate (lamina 



terminalis). It is continuous 



below with the anterior wall ^ 2, 



of the infundibulum of the 



between-brain. 



In the further develop- 

 ment of each vesicle of the 

 hemispheres four processes 

 are intimately associated : 

 ( 1 ) an extraordinary growth 



Fig. 251. Brain of a human embryo of three months, 

 after KO'LLIKER. Natural size. 



1. From above with the hemispheres removed and the 

 mid-brain opened. 2. The same from below. 

 /, Anterior part of the marginal arch (Randbogen) 

 of the cerebrum cut through ; /', posterior part 

 (hippocampus) of the marginal arch ; tho, optic 

 thalamus ; at, corpus striatum ; to, tractus opticus ; 

 cm, corpora mammillaria ; p, pons Varolii. 



and an enlargement in all 



directions resulting from 



it ; (2) an infolding of the 



wall of the vesicle, so that 



externally there arise deep 



clefts (the fissures), and 



internally projections into 



the lateral ventricles; (3) the development of a system of commissures, 



by means of which the right and left hemispheres are brought into 



closer union (corpus callosum and fornix>; (4) the formation of 



