NEAL: NERVOUS SYSTEM 1N SQUALUS ACANTHIAS. 173 
actually obliterated all surface evidence of such a condition. Each hind- 
brain neuromere, therefore, consists of a lateral thickening and a dorsal 
and ventral dilatation of the wall of the neural tube. The constrictions 
are exactly opposite on the two sides of the brain, The elongated cells 
are placed radially to an imaginary point situated in the middle of the 
thickening of the wall opposite. The nuclei are generally nearer the 
outer than the inner surface, and approach the latter only in the region 
of the constriction between the neuromeres. In this region the cells 
are more crowded, but the cells of one neuromere do not extend into 
the adjacent neuromeres, 
The hindbrain neuromeres, being structural differentiations of the 
walls of the neural tube, are not to be explained as the result of a 
simple mechanical process. The essential similarity of these serial 
groupings of nerve cells to the metameric ganglia of Annelids will, I 
believe, impress others as well as myself. A reconstruction of ‚he neu- 
romeres as they appear in this typical condition is shown in Figure 40, 
Plate 6. 
III. The Neuromeres in the Trunk Region. 
a. DEVELOPMENT OF MYELOMERES. 
It might seem that a more natural sequence in the study of neuro- 
meres than the one here followed would be to pass from the simpler 
conditions which obtain in the trunk to the more complicated ones in 
the head region. Instead of this, I follow the historical sequence, hav- 
ing begun with the “ Krauselungen,” or foldings, first seen by observers 
n the region of the hindbrain, and now pass to the study of the con- 
ditions in the spinal cord. That “hindbrain neuromeres” could be 
compared with segments of the spinal cord was an afterthought on 
the part of embryologists, evidently born of the conception that the 
ıead has a segmentation comparable with that in the trunk. 
While the neural plate in the trunk region is still widely open, its 
dorsal surface exhibits cross furrows, which are proved by longitudinal 
sections to correspond with the interspaces, or clefts, between the meso- 
dermic somites. The number of the cross furrows exactly equals that or 
the interspaces, increasing in number as the constrictions between the 
somites do. They do not, however, extend to the edges of the neural 
plate, but are restricted to the region where the plate rests upon the 
somites. In these cross furrows we have the first indications of those 
structures which were called by McClure (’89) “myelomeres,” and were 
