444 



EMBRYOLOGY. 



Fig. 253. Lateral viw of the brain of an embryo Calf 

 5 cm. long. The lateral wall of the hemisphere 

 has been removed. After MIHALKOVICS. Magni- 

 fied 3 diameters. 



cst, Corpus striatum ; ML, foramen of MONKO ; agf, 

 plexus choroideus lateralis ; amf, hippocampal 

 fold ; kh, cerebellum ; Dp, roof of the fourth 

 ventricle ; bb, pontal flexure ; mo, medulla ob- 

 longata ; mh, mid-brain (parietal flexure). 



(figs. 254 pi and 255 aaf). Instead of thickening and developing 



nerve-substance, it becomes 

 attenuated, and is trans- 

 formed into a single layer 

 of flat epithelial cells, which 

 are firmly united with the 

 pia mater. The latter then 

 becomes very vascular along 

 the entire fold, and grows 

 into the lateral ventricle in 

 the form of tufts, which 

 carry the epithelium before 

 them. In this way the 

 lateral choroid plexus arises 



(fig. 254 pi), which afterwards, in the adult, fills a part of the cella 



media and in- 

 ferior cornu. 



It begins at 



the foramen of 



M o N R o (fi g. 



253 JfZ),where 



it is continuous 



with the an- 

 terior unpaired 



choroid plexus 



which has 



arisen in the 



roof of the be- 



tween-brain 



vesicle. If the 



delicate vas- 

 cular pia mater 



is drawn out 



from the cho- 

 roid fissure, the 



wall of the 



brain, which is 



reduced to a 



Fig. 254. Transverse section through the brain of an embryo Sheep 



2 -7 cm. in length, after KOLLIKER. 



The section passes through the region of the foramen of MONEO. 

 at, Corpus striatum ; m, foramen of MONKO ; t, third ventricle ; pi, 



plexus choroideus of the lateral ventricle ;/, falx cerebri ; th, deepest 



anterior part of the optic thalamus; ch, chiasma ; o, optic nerve ; 



c, fibres of the crus cerebri; h, hippocampal fold ; p, pharynx; 



so,, presphenoid ; a, orbito-sphenoid ; s, part of the roof of the 



brain at the junction of the roof of the third ventricle with the 



lamina terminalis ; I, lateral ventricle. 



thin epithe- 

 lium, is at the same time destroyed, and there is produced in the 

 median wall of the hemisphere a gaping fissure, which extends from 



