NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS. 175 
The structure of the myelomeres in embryos of 40-50 somites is 
represented in Figures 42 and 43, Plate 6. As in Figure 39, only the 
right half of the embryo is shown. The only evidence of the structural 
peculiarities of neuromeres at this stage consists in an external constric- 
tion opposite the myotome and the spinal ganglion (Fig. 43), In 
sections dorsal or ventral to the one shown in Figure 43, even this con- 
striction becomes lost (see Fig. 42, which is more ventral, occupying the 
region of the voutral roots). All traces of an internal dilatation and 
constriction, and of the concomitant radial arrangement of cells, have 
disappeared. In the head, on the contrary, the “neuromeres ” still pre- 
serve all the characteristics seen in the earlier stages. 
An examination of the structure of the myelomeres shows that the 
conditions are easily explicable on the mechanical grounds stated. There 
are no serial thickenings of the wall of the neural tube, as in the hind- 
brain, and the radial arrangement of cells and nuclei shown in the frontal 
sections (Fig. 39) presents no difficulty ; for the cells composing the 
epithelium of the neural tube always have their long axes perpendicular 
to the surface of the tube, so that, if the tube becomes constricted oppo- 
site each somite, the cells will necessarily show a radial arrangement in 
frontal sections. In view of this fact, it is difficult to understand how 
investigators should have thought that the existence of a radial arrange- 
ment of cells and nuclei was evidence sufficient to establish the morpho- 
logical value of myelomeres, and their serial homology with hindbrain 
neuromeres, McClure (90), for example, says, “The lateral walls of the 
spinal cord are divided into neuromeres which, while less conspicuous, 
have all the cellular characteristics seen in the typical neuromeres of the 
hindbrain, and in fact are a continuation of the latter.’ That all of the 
cellular characteristics seen in the typical neuromeres of the hindbrain 
are also found in the myelomeres is demonstrably untrue for Squalus, as 
may be seen by comparing the sections shown in Figures 38 and 39, Plate 
6, both from the ventral half of the neural tube of the same embryo, one 
in the head and the other in the trunk. The cellular arrangements are 
decidedly unlike. Inthe head (Fig. 38) the cells and nuclei are crowded 
in the region of constriction between neuromeres, while in the trunk, 
if the cells are crowded at all, it is in the region of dilatation of the 
myelomere, 
It has seemed a strong argument for the serial homology of myelo- 
meres and hindbrain neuromeres that the former continue into the latter 
gradually and in an unbroken series. For example, McClure (*90) 
stated that “the constrictions of the myelon (in Lizard embryos) gradu- 
