NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS. 179 
Kupffer finds at least jive encephalomeres in the primary forebrain. 
This conclusion seems strengthened by the conclusion of Burckhardt (’93), 
that the median zones of the neural tube retain throughout the Verte- 
brate series the primitive segmentation best, and therefore are the best 
for comparison. 
My conclusions from a study of the evidence presented by those who 
have assumed a segmental value for the secondary subdivisions of the 
forebrain and midbrain vesicles are, (1) that morphologically different 
structures have been described by them as “ neuromeres ” or “ encepha- 
lomeres,” and (2) that the divergence in their results does not seem to 
justify this assumption. 
I now turn to an examination of the development of forebrain and 
midbrain regions in S. acanthias, in order to determine whether or not 
it is probable that structures morphologically comparable with hindbrain 
neuromeres exist in these regions. Since hindbrain neuromeres involve 
all three zones — dorsal, ventral, and lateral—of the walls of the 
encephalon, the value of forebrain and midbrain segments as morphologi- 
cal equivalents of them will clearly depend on their similarly involving 
those zones. If they do not, it is incumbent upon one who holds to 
their equivalency to demonstrate how modification has probably obscured 
or obliterated the primitive conditions. Evidence in such a highly 
specialized region can be at best only probable. Here, however, as 
always, the demonstration of morphological comparability must be 
“controlled” by the demonstration of similar relationships to other 
organ systems. 7 
b. DEVELOPMENT or THE FOREBRAIN AND MIDBRAIN. 
At a stage with 19 or 20 somites the conditions in the anterior brain 
region are very simple. The primary forebrain and midbrain are simple 
vesicles or enlargéments of the neural tube. A parasagittal section cut 
through the right wall of the neural tube is represented in Figure 45, 
Plate 7. Six vesicles are counted, all of them being included in the 
region of the cephalic plate. The anterior vesicle shown is the wall of 
the forebrain in the region of the optic vesicles. Behind lies the mid- 
brain, separated by a slight constriction from that region of the hind- 
brain to which Zimmermann ('91) has given the name “ Hinterhirn.” * 
Hindbrain neuromeres-IV, V, and VI are clearly defined. 
A frontal section of an embryo of the same stage, so cut as to coincide 
1 The English term hindbrain has been applied to the region separated by 
the Germans into “Hinterhirn” and “ Nachhirn.” 
