THE ORGANS OF THE OUTER GERM -LAYER. 431 



Whereas at the beginning of the development the mid-brain 

 vesicle (figs. 235, 243 mh), as a consequence of the curvature of the 

 neural tube, occupies the highest position and produces the parietal 

 prominence of the head (fig. 158 s), it is afterwards covered in from 

 above by the other parts of the brain, which are becoming more 

 voluminous, the cerebellum and cerebrum, and is crowded down 

 to the base of the brain (compare fig. 235 mh with fig, 241 vh). 



(4) Metamorphosis of the Second or JBetween-brain Vesicle. 



The between-brain vesicle also remains small, but undergoes a 

 series of interesting changes, since, apart from the optic vesicles, 

 which grow out from its walls, two other appendages, of proble- 

 matical meaning, are developed from it the pineal gland and the 

 hypophysis. 



In the case of the between-brain vesicle, it is only in the lateral 

 walls that a considerable amount of nerve-substance is formed. By 

 this means the walls thicken into the optic thalami with their 

 ganglionic layers. Between them the cavity of the vesicle is retained 

 as a narrow vertical fissure, known as the third ventricle ; it is 

 united with the 'fossa rhomboidalis by means of the aqueduct of 

 SYLVIUS. The floor remains thin and at an early period becomes 

 evaginated downwards ; it thus acquires the form (figs. 235, 241 tr) 

 of a short funnel (infundibulum}, with the apex of which is united 

 the hypophysis, soon to be fully described. 



The roof presents in its metamorphosis a striking similarity to the 

 corresponding part of the after-brain vesicle (fig. 241). It persists 

 as a simple, thin epithelial layer, unites with the very vascular 

 pia mater, which sends out in this case also villous outgrowths 

 with capillary loops which pass into the third ventricle, and together 

 with it constitutes the anterior choroid plexus (tela choroidea anterior 

 or superior). When in withdrawing the pia mater the choroid 

 plexus is also removed, the third ventricle is opened ; thus is produced 

 the anterior great fissure of the brain through which, as through 

 the structure of the same name in the medulla oblongata, one can 

 penetrate into the cavities of the brain. 



The agreement with the medulla oblongata is expressed in still 

 another point. As in the case of the latter the edges of the roof- 

 plate develop into thin medullary bands, by means of which the 

 attachment to the sides of the fossa rhomboidalis is accomplished, so 



