NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS. 169 
being exactly frontal on account of the torsion of the embryo. The lat- 
eral walls of the neural tube are seen in the figure to be thickened in 
that region which lies just posterior to the constriction opposite van 
Wijhe’s 3d somite. A comparison of many frontal and sagittal sections 
leaves no doubt that this thickening lies in the region of neuromere IV. 
That expansion of the neural tube which lies between the 2d and 3d 
somites, and which is separated by an external constriction from neuro- 
mere IV behind and from the midbrain vesicle (encephalomere II of my 
figures) in front, is the most anterior of the primary expansions or 
encephalomeres of the hindbrain. It has been called by Zimmermann 
C91) “ Hinterhirn.” This corresponds to the third expansion of the 
neural tube in the chick (Fig. 44, Plate 7), as may be determined by its 
relation to the acustico-facialis Anlage and the auditory invagination. 
Failure correctly to identify this vesicle in the chick led Miss Platt (89) 
to call the second vesicle, viz. the primary midbrain, the hindbrain. 
At a later stage, when 17 to 18 somites are differentiated, a well 
marked local thickening in the posterior half of encephalomere III 
appears.! A frontal section of an embryo at this stage, showing neuro- 
mere IV as a local thickening posterior to neuromere III, is seen in 
Figure 23, Plate 5. Encephalomere III is separated by a constriction 
from encephalomere II. At this stage, then, only four of the hindbrain 
neuromeres (III, IV, V, and VI) are differentiated, and the conditions 
remain the same when one more somite is formed. 
In a similar frontal section of an embryo with 19 somites, such as is 
represented in Figure 24, four symmetrical thickenings of the lateral 
walls of the hindbrain (III-VI) appear. Opposite neuromere V lie 
the cells of the Anlage of the acustico-facialis nerve (blue), and opposite 
neuromere VI the thickened auditory epithelium. Neuromere VII is 
not present at this stage, and it does not begin to be differentiated until 
after one or two more mesodermal somites are formed, when a faintly 
marked dorsal and ventral dilatation appears in the region of the neu- 
ral tube just behind neuromere VI (Fig. 9, Plate 3). The lateral walls 
of this neuromere never become so markedly thickened as the walls of 
the other neuromeres, nor does the neuromere show a constriction at its 
posterior border before the embryo reaches the condition of Balfour's 
stage H, and then only a faintly discernible one. A cross section 
1 Such a secondary subdivision ot encephalomere III. (“ Hinterhirn ”) occurs in 
the chick as in S. acanthias. I regard the primary vesicle as of different morpho- 
logical value from that of its subdivisions, for reasons which will be made more 
apparent when the relations of the vesicles are studied. 
