AMPHIBIAN NERVOUS SYSTEM 375 



the notochord is absent and the myotomes join one another in the 

 middle hne beneath the neural tube, the latter has a very thick 

 floor and thin roof, and the central canal extends horizontally instead 

 of vertically (fig. 180; see also p. 220). 



It is clear, therefore, that the normal bilateral symmetry of the 

 neural tube is dependent on the presence and normal relative 

 positions of notochord and myotomes. Not only does the notochord 

 induce the formation of the neural tube (p. 135), but it plays a part 

 in determining its subsequent differentiation. 



If a portion of neural tube is made to develop in a region of 

 mesenchyme (i.e. deprived of the proximity of notochord and 

 myotomes) it differentiates with radial symmetry : the walls are of 

 equal moderate thickness, and the central canal is of circular cross- 

 section.^ 



It follows that an environment of myotomes is unlikely to be 

 conducive to the formation of the vertebrate brain and its numerous 

 outgrowths and vesicles, and, in point of fact, myotomes are absent 

 from the neighbourhood of the fore-brain, where the somites are 

 destined to become the extrinsic eye-muscles. Conversely, in 

 AmphioxuSj where myotomes flank the neural tube right up to its 

 anterior end, there is a minimum of cerebral differentiation. 



The conditions of the histological differentiation of the neural tube 

 must now be considered. This consists of the formation in definite 

 regions of accumulations of the cell-bodies of the neurons or nerve- 

 cells forming the grey matter, and of the development from these 

 cells of axons or fibres in definite directions or tracts forming the 

 white matter. The subsequent morphological differentiation of the 

 brain is really only the result of the histological differentiation of 

 neurons in particular places, and the directed growth of their axons. 

 The prefacial, postfacial, and hemispheric centres in the brain, and 

 the anterior region of the spinal cord, appear to be places at which 

 a certain definite number of neurons are determined at the early 

 neural tube stage to develop by self-differentiation.^ The proof of 

 this for the centre in the anterior region of the spinal cord is given 



^ Holtfreter, 1933 b. 2 Detwiler, 1925 b; Coghill, 1929. 



