334 



THE MORPHOGENESIS OF THE CENTRAL NERVOUS SYSTEM 



337 A). The cavity of the myelencephalon is thus widened from side 

 to side, and flattened dorso-ventrally. This is most marked cranially, 

 where, between the alar plates of the myelencephalon and metencephalon , 

 are formed the lateral recesses of the fourth ventricle (Figs. 337 and 353) . 

 Into the ependymal roof of the myelencephalon blood vessels grow, and, 

 invading the lateral recesses, form there the chorioid plexus of the fourth 

 ventricle. The plexus consists of small, finger-like folds of the ependymal 

 layer and its covering mesenchymal layer. The line of attachment of the 

 ependymal layer to the alar plate is known as the rhombic lip and later 

 becomes the tccnia and obex of the fourth ventricle (Fig. 337 B). 



Mid-brain 



Cerebellum 



Lateral recess 



Rlwmbic lip 



Lateral lobe of cerebellum Lobules of vermis 



Flocculus 



B Lateral lobe of Pyramis 



Corporaquadrigemina. -^ cerebellum 



Cerebrum 



Anlage of 

 vermis 



Lateral lobe of 

 cerebellum 



Rhombic lip ' W ^ m ^ mi .Obe X Flocculu / ^ \ ^ Umla 



Nodulus 



FIG. 337. Dorsal views of four stages in the development of the cerebellum. A, 13.6 mm. 

 (His); B, 24 mm.; C, no mm.; D, 150 mm. 



In early stages the floor of the myelencephalon is constricted trans- 

 versely by the so-called rhombic grooves, six in number; the intervals be- 

 tween successive grooves are neuromeres (cf. Figs. 96 and 122). Some have 

 have viewed these as evidential of a former segmentation of the head 

 similar to that of the trunk. It is more probable, however, that they 

 merely stand in relation to certain cerebral nerves and hence their seg- 

 mental arrangement is secondary. 



The further growth of the myelencephalon is due: (i) to the rapid 

 formation of neuroblasts, derived from the ependymal and mantle layers ; 

 (2) to the development of nerve fibers from these neuroblasts; (3) to the 



