168 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 
later stages, whereas it will be shown in Squalus that this is not the 
case. 
The want of abundant material of early stages felt by investigators in 
most cases is not a hindrance in the case of Squalus, for the early stages 
are as easily obtained as the later ones. In the study of the develop- 
ment of neuromeres, I have made use, first, of specimens very lightly 
stained in hematoxylin and mounted in toto in balsam, and secondly 
of the usual cross, frontal, and sagittal sections. The series of embryos 
represented in Plate 3 is chiefly of value in showing the neuromeres in 
successive stages, and the relations of the masses of cells composing 
the neural crest, or ganglionic Anlagen (colored blue in the figures), as 
seen in cleared specimens. The neural tube is represented as seen in 
optical section, while the other structures of the right half of the embryo 
are projected upon the median plane. 
The earliest evidence of hindbrain neuromeres which I have found is 
seen in embryos of 14 or 15 somites in which the cephalic plate has not 
closed in the hindbrain region. In most embryos with that number of 
somites the plate is already closed, but in cases where it has not, neuro- 
meres IV, V, and VI are seen as thickenings of the lateral walls of the 
hindbrain before its closure. Usually closure takes place, as in the 
chick, first in the region of the so called trigeminus Anlage, and later in 
the region of neuromere V, the most anterior portion of the cephalic 
plate remaining open as the neuropore until considerably later stages. 
Figure 7, Plate 3, shows that in embryos of 14 to 16 somites (in the 
specimen figured, after the closure of the cephalic plate) four expansions 
of the neural tube in the hindbrain region are differentiated (neuromeres 
Ill, IV, V, VI). The hinder boundary of neuromere VI marks the for- 
mer posterior boundary of the cephalic plate. The figures show (and this 
is a point of considerable importance in considering the morphological 
value of neuromeres) that each neuromere corresponds to the region of 
a dorsal as well as a ‘ventral expansion of the neural tube, and that the 
neuromeres are ‘separated from one another by both dorsal and ventral 
constrictions, which are to be seen both in sagittal sections and in 
cleared specimens. 
Frontal sections at this stage give additional evidence concerning the 
structure of hindbrain neuromeres. A frontal section just below the 
axis of the neural tube is shown in Figure 22, Plate 5. The section 
shows that the cephalic plate is still open in the region of the forebrain. 
The dorsal portion of the mesoderm in the region of van Wijhe’s 2d and 
3d head somites (2 and 3) is cut on the right side only, the sections not 
