374 



THE FURTHER DIFFERENTIATION OF THE 



hemispheres is now determined to evaginate to form vesicles,^ and 

 the eye-cup, with its stalk, retina, and tapetum, becomes quahta- 

 tively and quantitatively determined. In the remainder of the neural 

 tube, centres of differentiation of neurons from the neuro-epithelial 

 cells, and of their greater or lesser degree of proliferation, are deter- 

 mined at certain definite places.^ The main lines of the regional 

 determination of the nervous system are thus completed when the 

 neural folds have fused with one another to give rise to the neural 

 tube, and the optic cups have been formed. 



a b c 



Fig. 1 80 

 Diagram showing the effect on the differentiation of the neural tube of a, proxi- 

 mity of a notochord without myotomes; b, proximity of myotomes without 

 notochord ; c, absence of notochord and myotomes (mesenchymal environment) ; 

 as seen in transverse section. (From Holtfreter, Arch. Entwmech. cxxvii, 1933.) 



At the same time, certain features of the differentiation of the 

 neural tube are not independent of the presence of other structures. 

 For instance, the notochord is responsible for the formation of the 

 ventral sulcus of the central canal, i.e. it determines the formation 

 of a thin floor on the side of the neural tube immediately overlying 

 it.^ On the other hand, the myotomes which flank the neural tube 

 are responsible for the formation of the thick lateral walls, and for 

 the radial arrangement of the cells in them^. These facts emerge 

 from experiments in which embryos were obtained possessing a 

 notochord but no myotomes, or with myotomes but no notochord. 

 It will be noticed that the action on the differentiation of the neural 

 tube of both notochord and myotomes tends to the same result. If 



Nicholas, 1930. 



Coghill, 1929; Detwiler, several works. 



^ Mangold and Seidel, 1927; Bautzmann, 1928; Bytinski-Salz, 1929. 

 * Lehmann, 1926. 



