170 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
through neuromere IV, which serves to show how greatly thickened 
the lateral wall is at this stage, is shown in Figure 32, Plate 5. The 
dorsal wall of this neuromere is considerably thicker than that of the 
neuromeres anterior and posterior to it, possibly because few cells are 
proliferated from this neuromere to form the ganglionic Anlage or neu- 
ral crest. 
I pass now to a description of the hindbrain neuromeres (encephalo- 
meres) at a stage with 28 or 30 somites (Balfour’s stage H). Since at 
this stage the neuromeres are clearly differentiated, and the thinning and 
expansion of the roof of the hindbrain have progressed very little, this 
is a most favorable stage for the study of the structural and histological 
peculiarities of the hindbrain neuromeres.! Figure 13, Plate 3, repre- 
sents a cleared specimen at this stage, and Figure 25, Plate 5, a frontal 
section of the same. Opposite neuromere III (Fig. 25) lies part of the 
trigeminus Anlage ; opposite neuromere V lie the cells of the acustico- 
facialis Anlage ; and opposite neuromere VI lies the thickened auditory 
epithelium, which is just beginning to invaginate. The acustico-facialis 
Anlage always remains in relation with neuromere V, so that this serves 
as an excellent starting point in counting the neuromeres. In order to 
get a clear conception of the structure of the neuromeres, cross, frontal, 
and sagittal sections are necessary. The series represented in Figures 
36-38, Plate 6, are frontal sections taken at different levels (a, B, y, 
Fig. 40, Plate 6) in the medullary tube. Only the right wall of the 
medullary tube in the region of neuromeres IV and V is shown in de- 
tail. The first section (Fig. 36) is dorsal, in the region of the “ Deck- 
platte.” In this section it is seen that what Orr (’87) has said for the 
Lizard (see page 167) is true for Squalus. The section reproduced 
in Figure 37, more ventral than Figure 36, shows that the conditions 
which obtain in the region of the lateral zones are somewhat different 
from those of the'dorsal zone. Since no sharp internal ridge exists, each 
lateral half of a neuromere does not appear in section as an arc of a 
circle, but as a thickening of the wall of the medullary tube. The cells 
and nuclei are fewer in number and more crowded in the region of con- 
striction between neuromeres. Although there is no inner concavity at 
this level, the cells and nuclei (Fig. 37) show radial arrangement sim- 
ilar to that shown in Figure 36. The ventral section (Fig. 38) differs 
in no essential respect from the dorsal one. I have chosen these two 
neuromeres (IV and V) for description, since they with neuromere VI 
1 The head somites, likewise, appear at this stage most clearly differentiated. 
It is, in fact, the “acranial stage ” of the embryo. 
