444 



EMBRYOLOGY. 



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

 m h 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 j)l), which afterwards, in the adult, fills a part of the cella 



media and in- 

 ferior cornu. 



It begins at 



the foramen of 



MONRO (fig. 



Fig. 253. --Lateral view 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 MOXRO ; agf, 

 plexus choroideus lateralis ; amf, hippocampal 

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

 ventricle ; 66, pontal flexure ; mo, medulla ob- 

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



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 

 thin epithe- 

 lium, is at the 

 median wall of 



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



2-7 cm. in length, after K.OLLIKER. 



The section passes through the region of the foramen of MONP.O. 

 si, Corpus striatum ; TO, foramen of MONRO ; t, third ventricle ; pi, 



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



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



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



sa, pyesphenoid ; 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, 



same time destroyed, and there is produced in the 

 the hemisphere a gaping fissure, which extends from 



