THE ORGANS OF THE OUTER GERM-LAYER. 429 



and tli.- not' on the other. The former (figs. UM. _rj) are con- 

 siderubly thickened liy tin* addition of M.-I ;<-o and become 



separated on either side of tin- body (in .Man in the thii-.l b 

 M\th month-') info columns, which are recognisable from the out>id- 

 because they aro separated l.y ^ronves ; these are the extensions with 

 certain modification- of the three familiar columns of the spinal 

 cord. The roof of the vesicle (11- - ""> /;/' and fig. 243 Dp), on the 

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

 becomes still thinner, and in the adult consists of a single layer of 

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

 vent rally 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 ch 

 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-bruin 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 in 

 of thin bunds of white nervous substance, which, us obex, taenia 

 MUMS rhomboidalis, velum medullnre postering, and peduncnlus 

 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/o///-// -.tide undergoes a considerable thick- 



ening in all it.- parts, and Mil-rounds it- cavity in the form of a 

 differentiated into several reyions ; the cavity becomes the anterior 

 part of the fossa rhomboidalis (figs. 243, '2-i'J, I'll). The floor 

 i'urni.-hes the pons (bb), the cross fibres of which become evident in 

 the fourth month. From the lateral walls ari-e the pedunculi 

 cerebelliad pontem. lint it i* t/t? roof that grows to an extraordinary 

 t cii'f i/'rex to the cerek'Hi'in its characteristic stamp. At first 



