NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS. 245 



and spinal cord of snake embryos not explicable on mechanical grounds. 

 " The neuromeres of the medulla cannot be ascribed to the mechanical 

 influence of the Anlageu of the nerves, for those segments which have 

 no nerves develop equally with the others" (cf. Froriep, '91). He 

 considers however " the neuromeres of the forebrain " region wholly 

 illusory from a morphological standpoint, since they involve only dorsal 

 structures. 



c. Interpretation op Hindbrain Neuromeres in Squalus acanthias. 



I believe that the evidence which I have obtained from a study of the 

 development of hindbrain neuromeres in S. acanthias excludes the 

 possibility of a simple mechanical explanation of them. In their earlier 

 stages they were seen to be local thickenings of the lateral walls, a 

 phenomenon intelligible only on the ground of unequal growth, and not 

 in the least explicable as the result of the passive bending or shoving 

 of a tube already formed.^ Since the somites do not extend into the 

 region of the dorsal part of the encephalomeres, the possibility that the 

 neural tube in this region is constricted by them is excluded. They are, 

 then, in both structure and mode of development, clearly not to be ex- 

 plained in the same way as the myelomeres. Again, that they are not 

 due to the effect of the Anlagen of the nerves, as supposed by Froriep, 

 is shown by the fact that encephalomere IV develops equally with the 

 others, although there is no nerve in relation with it until a compara- 

 tively late stage. Since tlie fibrillar connection of nerves with neui'o- 

 meres is established almost at the same time that the inner surface of 

 the hindbrain neuromeres becomes concave, it might be thought that this 

 change is due to the mechanical effect of nerve fibres. That such is not 

 the case seems clear, however, because no nerve fibres come into relation 

 with the outer convexity of encephalomere VI. The hindbrain neuro- 

 meres, from their early appearance onwards until they disappear, are 

 local diff"erentations of the walls of the medulla, and as such are not, I 

 believe, to be satisfactorily explained on simple mechanical grounds. On 

 the other hand, I hold that they do possess certain characteristics which 

 admit of a mechanical explanation. This seems to be supported by evi- 

 dence from two sources. lu the first place, a fixing agent which causes 

 a contraction of the tissues of the embryo intensifies the constrictions 

 between the neuromeres. By this means the radial arrangement of cells 



1 This is true also in swine and chick embryos ; but I do not find in Ambly- 

 stoma as good evidence that the neurometes are local thickenings of the neural 

 wall. 



