DEVELOPMENT OF RANA 181 



neural plate does not get folded into the nerve-tube when the 

 neural folds meet. It lies just to the side of the point of fusion 

 of the neural folds, and forms the neural crest. The cells of 

 the neural crest are destined to give rise to the afferent sensory 

 nerve-cells, whose cell-bodies form the ganglia on the dorsal 

 roots of the nerves, and to the sheaths of the nerves. 



Segmentation. — The mesoderm on each side of the nerve- 

 tube and notochord becomes thickened and divided into blocks, 

 which are the somites from which the myotomes develop ; 

 they are metamerically segmented. This segmentation begins 

 anteriorly and proceeds backwards ; but it does not affect 

 the more ventrally-situated mesoderm. Whereas the dorsal 

 portion of the ccelomic cavity (on a level with the myotomes) 

 is interrupted by transverse septa separating the mesodermal 

 somites from the somites in front and behind, and consists of 

 a number of myoccels equal to the number of somites, the 

 ventral portion of the ccelomic cavity is continuous and un- 

 interrupted by septa. 



The segmented region of the mesoderm is called the verte- 

 bral plate, the unsegmented portion is the lateral plate. 

 Between each somite and the lateral plate immediately below 

 it is a small region of segmented mesoderm known as the inter- 

 mediate cell-mass, or nephrotome. From these structures 

 the tubules of the kidneys will arise, and they are therefore also 

 segmental. Eventually, the vertebral plate separates completely 

 from the lateral plate, and the myotomes grow down in the body- 

 wall lateral to the splanchnoccel to give rise to the muscles of 

 the ventral surface, of the limbs, and the hypoglossal muscula- 

 ture beneath the mouth. 



Muscles formed from myotomes are always innervated by 

 ventral nerve-roots, and as the myotomes are segmental, the 

 ventral nerve-roots which grow freely out from the nerve-tube 

 are segmental also. Further, the neural crest becomes sub- 

 divided into pieces corresponding to the myotomes, these are 

 the rudiments of the dorsal-root ganglia. The cells in these 

 ganglia develop one process which grows into the nerve- tube, 

 and another which pushes out to its destination in the body. 

 These dorsal nerve-roots are therefore segmental also. 



