NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS. 183 
Two frontal sections of an embryo at this stage are seen in Figures 51 
and 52, Plate 7. Anteriorly in the more dorsal section (Fig. 51) is seen 
the expansion of the prosencephalon. Behind this lies an expansion 
which might be considered as a neuromere, if a radial arrangement of 
nuclei and a constriction of the brain wall were alone considered suffi- 
cient criteria for such a structure, Since, however, it is simply a dorsal 
expansion, which is unrelated to nerves, and soon becomes differentiated 
into adult organs, I am unable to regard it as a neuromere. From it are 
differentiated “ Zirbelpolster ” (parencephalon, Nebenhirn, or Zwischen- 
hirnblase) and epiphysis. Posterior to the constriction marked by 
the arrow, which corresponds with the point so marked in Figure 50, 
is situated a long expanded portion of the encephalon which passes with- 
out constriction into the midbrain vesicle. In the more ventral section 
(Fig. 52), however, there is seen in this region a constriction which may 
be traced ventrally to that point from which the anterior root of the 
oculomotor arises. Two neuromere-like expansions, separated by the 
constriction between primary forebrain and midbrain, are seen in this 
stage as in the previous stage described. 
Passing now to a much later stage (21-22 mm.), we find (Plate 4, 
Fig. 21) that the: posterior commissure has come to lie much nearer the 
base of the stalk of the epiphysis, and thus that the portion of the dor- 
sal wall which is called by Kupffer diencephalon has become much re- 
duced in the region of the midbrain vesicle. Thus it has come about 
that frontal sections in a plane midway between the dorsal and the ven- 
tral walls of the neural tube (Fig. 53, Plate 7) show only a single neuro- 
mere-like expansion. In more dorsal as well as more ventral sections 
this undergoes constriction, so that it is by no means a simple neuro- 
meric enlargement. A median sagittal section, such as that shown in 
Figure 21 (Plate 4), is the most satisfactory for the study of segmenta- 
tion at this stage. The primary forebrain is now differentiated into the 
successive dorsal dilatations epencephalon, paraphysis (parencephalon), 
and epiphysis. Dorsally the midbrain still continues to be a simple 
expansion, while ventrally traces of the three segments still remain, 
the anterior one having become much reduced in length. 
With the exception of Locy, Zimmermann (’91) is the only investigator 
who has studied the “neuromeres” in Selachii. For the purpose of 
comparison, it is well to stave his results here. He finds at first eight 
“primäre Abschnitte” in the encephalon, the first three of which 
exceed in size the last five. The first three are the Vorderhirn, Mittel- 
hirn, and Hinterhirr each of which he regards as a complex of en- 
