418 



EMBETOLOGT. 



lanceolatus only ; in all other Vertebrates, on the contrary, it is 

 differentiated into spinal cord and brain. 



(a) The Development of the Spinal Cord. 



The part of the neural tube which is converted into the spinal 

 cord is oval in cross section (fig. 200). At an early period a separa- 

 tion into a right and left half can be recognised (fig. 232). For 



Fig. 832 — Cross section of an embryo Lizud with completely closed intestinal tube, after 

 Sagemehl. 



he, Posterior, vc, anterior commissure of the spinal cord ; vw, anterior root of nerve ; «/, nerve- 

 fibriUse ; spk, spinal ganglion ; mp', muscle-pjate, musole-forming layer ; mp', outer layer of 

 the rauscle-plate ; mjj", transition of the outer into the muscle-forming layer. 



the lateral walls are greatly thickened and consist of several layers 

 of long, cylindrical cells, whereas the upper and lower walls are thin 

 and can be distinguished respectively as posterior [dorsal] and anterior 

 commissure {ho and vc), or as roof-plate and floor-plate. 



The further development, of which I shall mention only the most 

 important points, takes place in such a manner that the lateral 

 halves become thicker and thicker (fig. 233). The cells continue to 

 increase in number by division, and at the same time to be differ- 

 entiated into two histological groups — (1) into elements which provide 

 the sustentative framework, the epithelium surrounding the central 



