Chapter XI 



THE FURTHER DIFFERENTIATION OF THE 

 AMPHIBIAN NERVOUS SYSTEM 



§1 



The differentiation of the amphibian nervous system presents a 

 number of special problems of great interest for the physiology of 

 development. A large number of experiments have been made on 

 this subject, and they illustrate so many of the principles which 

 operate to bring about differentiation, that a chapter may be pro- 

 fitably devoted to it. 



At the blastula stage, as already mentioned, the presumptive 

 neural fold material occupies a zone in the form of a transverse 

 band, at right angles to the plane of bilateral symmetry, and passing 

 close to the animal pole of the egg. This presumptive neural fold 

 region appears to have received a partial and labile determination in 

 situ before gastrulation, and this is more marked in the region of the 

 brain than in that of the spinal cord. Then, during gastrulation, a 

 streaming movement of the cells of the animal hemisphere takes 

 place, which results in a shifting of the presumptive neural fold 

 material, so that it comes to occupy the position of a band running 

 down the dorsal side of the embryo. At the same time, the 

 organiser has become invaginated, and having become the noto- 

 chord, gut-roof and mesoderm, it underlies the neural fold region 

 and determines it irrevocably to develop by self-differentiation. 

 As already mentioned (p. 28) the definitive neural tube arises 

 from the anterior 4/5th of the neural folds, while the hindmost 

 fifth becomes caudal mesoderm. 



The definitive determination of the neural fold field as a whole does 

 not prevent the possibility of a considerable degree of regulation 

 taking place within it. This implies, as explained above (Chap, vii, 

 p. 239), that its various constituent structures have not yet been 

 individually localised, delimited, and determined. Such further 

 determination soon follows, however; the region of the cerebral 



