NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHUS. 247 



tube in a confined sjjace. Such gi'owth would clearly result in a shoving 

 of the neural tube, and also in a flexure in weaker portions, as in the 

 regions between local thickenings, like the hindbrain neuromeres. The 

 crowding of the cells in the regions of constriction between neuromeres 

 may be accounted for in the same way. I therefore conclude that some 

 of the structural characteristics of ueuromei'es may be intensified by the 

 bending or shoving of the neural tube during its growth. 



The conditions presented in Amblystoma (Plate 5, Fig. 35) led me to 

 believe at one time that the neuromeres might be related to the prolifera- 

 tion of the cells of the ganglionic Anlage. In this animal the neural 

 tube is evaginated in the regions of the proliferation of cells for the 

 ganglionic Anlagen of nerves V, VII, IX, and X, while in the region 

 where no neural-crest cells are proliferated — the region corresponding 

 to the position of encephalomere IV (in other forms) — no neuromei'e 

 appears.' In S. acanthias we have seen (page 215) that from two of the 

 hindbrain neuromeres, viz. V and VI, are proliferated the cells of two 

 distinct nerve Anlagen. But since no nerve Anlage is proliferated from 

 encephalomere IV, although this is as well marked as other encephalo- 

 meres, I was compelled to abandon the hypothesis, to which the study 

 of Amblystoma had inclined me. The fact that particular nerve Anlagen 

 are proliferated from particular encephalomeres may, however, he a clue to 

 the primitive mutual relationships of these nerves and of the encephalomeres 

 to each other. The fact that the local thickenings are confined to that 

 region of the neural tube from which the great nerves of the head — V, 

 VI, VII, IX, and X — arise, must also give us some clue as to their signifi- 

 cance. Such local thickenings are seen neither in the region anterior, nor 

 in that posterior to the medulla, but they are not limited by the ear cap- 

 sule posteriorly, and the anterior boundary of them does not coincide with 

 the anterior boundary of the primary hindbrain vesicle. It is to their 

 nerve relations, then, that an investigator must first turn his attention. 

 We have seen that in the development of the neural crest some of the 

 cells of the trigeminus are proliferated from encephalomere III ; that few 

 cells are proliferated from encephalomere IV: that from encephalomere V 

 come the cells of the acustico-facialis, from encephalomere VI the cells 

 of the glossopharyngeus, and from encephalomere VII the cells of the 

 Urvagus. The clearly marked relations of the Anlagen of the two suc- 



1 The migration of cells from certain regions of the neural tube would certainly 

 weaken these regions, and the tube would in consequence, if subjected to a longi- 

 tudinal pressure, or to distention by growth, tend to bend or distend most readily 

 in such places. 



