THE ORGANS OF THE OUTER GERM-LAYER. 429 



and the roof on the other. The former (figs. 241, 242) are con- 

 siderably thickened by the addition of nervous substance and become 

 separated on either side of the body (in Man in the third to the 

 sixth months) into columns, which are recognisable from the outside 

 because they are separated by grooves ; these are the extensions with 

 certain modifications of the three familiar columns of the spinal 

 cord. The roof of the vesicle (fig. 235 rf and fig. 243 Dp), on the 

 contrary, produces no nerve-substance, retains its epithelial structure, 

 becomes still thinner, and in the_adult^onsists of a single layer of 

 flat cells. This forms the only covering to the cavity of the dorso- 

 ventrally compressed vesicle of the after- brain the fourth ventricle 

 or fossa rhomboidalis. It is firmly applied to the under surface of 

 the pia mater, and with it produces the posterior choroid plexus (tela 

 choroidea inferior). The name choroid plexus has been chosen 

 because the pia mater in this region becomes very vascular and in 

 the form of two rows of branched villi grows into the cavity of the 

 after-brain vesicle, always carrying before it, and thus infolding, the 

 thin epithelial roof. 



Laterally the roof-plate or the epithelium of the choroid plexus is 

 continuous with the parts of the brain- vesicle that have been meta- 

 morphosed into nervous matter. The transition is effected by means 

 of thin bands of white nervous substance, which, as obex, taenia 

 sinus rhomboidalis, velum medullare posterius, and pedunculus 

 flocculi, surround the edge of the fossa rhomboidalis. If with the 

 pia mater one strips off from the medulla oblongata the posterior 

 medullary velum, the epithelial covering of the fourth ventricle 

 adhering to the latter will naturally be removed with it. In this 

 way is produced the posterior brain-fissure of the older authors, 

 through which one can penetrate into the system of cavities in the 

 brain and spinal cord. 



(2) Metamorphosis of the Fourth Brain-Vesicle. 



The wall of the fourth brain-vesicle undergoes a considerable thick- 

 ening in all its parts, and surrounds its cavity in the form of a ring 

 differentiated into several regions ; the cavity becomes the anterior 

 part of the fossa rhomboidalis (figs. 243, 242, 241). The floor 

 furnishes the pons (bb), the cross fibres of which become evident in 

 the fourth month. From the lateral walls arise the pedunculi 

 cerebelli ad pontem. But it is the roof that grows to an extraordinary 

 extent and gives to the cerebdlum its characteristic stamp. At first 



