334 



THE MORPHOGENESIS OF THE CENTRAL NERVOUS SYSTEM 



cephalon 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 epen- 

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

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



CeiehiU. 



Lateral recess 



Rhombic lip 



Corpora quadrigemina 

 Cerebrum 



Mid-brain 



Lateral lobe oj cerebellum Lobules of vermis 



f Medulla 

 oblongata 



Flocculus 



D 



Later all obe of Pyramis 

 ctrtbtlUim 



Lateral lobe of 

 cerebellum 



Rhombic lip 



Obex 



rio(i III us 



Nodulus 



Fig. 337.— Dorsal views ot four stages in the development of the cerebellum, yl, of a 13.6 mm. em- 

 bryo (His); B, of a 24 mm. embryo; C, of a 110 mm. fetus; D, of a 150 mm. fetus. 



In early stages the floor of the myelencephalon is constricted transversely by 

 the so-called rhombic grooves, six in number; the intervals between successive 

 grooves are neuromeres (cf. Figs. 96 and 122). Some have viewed these as evi- 

 dential 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 segmental arrangement is secondary. 



The further growth of the myelencephalon is due : (1) to the rapid formation 

 of neuroblasts, derived from the ependymal and mantle layers; (2) to the devel- 

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



