134 OUTLINES OF CHORDATE DEVELOPMENT 



The broad roof of the myelencephalon is wholly non-nervous 

 and forms the folded choroid plexus of the fourth ventricle (Fig. 

 42, B). The floor and ventro-lateral walls of the rhomben- 

 cephalon become greatly thickened, forming chiefly the nervous 

 pathways extending between the cord and brain and the nuclei 

 of origin of most of the cranial nerves. Posteriorly the myelen- 



FIG. 43. Transverse sections through the spinal cord of R. fusca. From Von 

 Kupffer (Hertwig's Handbuch, etc.). A. Through the anal region of a larva of 

 7 mm. B. Through the anterior body region of a larva during metamorphosis. 

 a, Spinal artery; c, central canal; d, dorsal column (white substance); dw, dorsal 

 root of spinal nerve; dz, atrophied dorsal cells; g, gray substance; vz, ventral 

 cells; w, dorso-lateral column (white substance). 



cephalon narrows gradually and passes insensibly into the spinal 

 cord. 



The spinal cord is at first flexed posteriorly toward the blasto- 

 pore (Fig. 37), but when the tail begins to grow out this flexure 

 disappears. The cavity of the cord is the central canal; it is 

 lined with a layer of non-nervous cells known as ependymal cells, 

 while the true nerve cells composing the greater part of its wall 



