444 



EMBRYOLOGY- 



(figs. 254 pi and 255 atf). 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 aris; s 



(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 



MONIIO (fig. 



253 Jf/^where 



it is continuous 



with the an- 



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 MONRO ; agf, 

 plexus choroideus lateralis ; am/, hippocampal 

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

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

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



'(/ 



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 KOLLIKEK. 



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

 st, Corpus striatum; in, foramen of MOXRO ; t, third ventricle; pi, 



plexus choroideus of the lateral ventricle ;/, f alx 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; 



sa, picsphenoid ; a, orbito-sphenoid ; s, part of the roof of the 



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



lamii:a 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 



