NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS. 247 
tube in a confined space. Such growth 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 neuromeres 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 neuromere 
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, be 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 TV; 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. 
