444 



EMBRYOLOGY. 



mh 



(figs. 254 pi and 255 oaf). 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. 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. 

 ust, Corpus striatum ; ML, foramen of MOXRO ; agf, 



plexus choroideus lateralis ; amf, hippocampal 



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



ventricle ; 66, pontal flexure ; 7710, medulla ob- 



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



(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 

 MONRO (fig. 

 253 JlfZ/\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 

 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 



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



27 cm. in length, after KOLLIKEK. 



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

 st, Corpus striatum ; TO, foramen of MONRO ; 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; 



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



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



lamina terniinalis; I, lateral ventricle. 



