NEAL, — Nervous Syst. Squalus. 
PLATE 7. 
Figures 44 to 53 illustrate the primary and secondary subdivisions of the forebrain 
and midbrain (encephalomeres I and II). All the Figures, except 47, 48, 53, 55, and 
56, magnified 43 diameters. 
Fig. 44. A parasagittal section of a Chick embryo of 33 hours’ incubation (14 
somites). Seven primary expansions of the encephalon appear, from 
the fifth of which, as in Squalus, the Anlage of the acustico-facialis is 
proliferated. 
Fig. 45. A parasagittal section of a Squalus embryo with 18 somites. Six primary 
vesiculations (encephalomeres) are seen, all included in the region of the 
cephalic plate. Clefts in the dorsal mesoderm separate from each 
other all of van Wijhe’s somites except the 4th and 5th. 
Fig. 46. A parasagittal section of a Squalus embryo with 28 to 80 somites. Both 
encephalomeres II and III have become subdivided by constrictions, 
that of the former, however, affecting the ventral wall only. All of 
van Wijhe’s somites separated by clearly marked mesodermie clefts. 
Fig. 47. A parasagittal section of an embryo with 65 somites, magnified 23 
diameters. Two subdivisions of encephalomeres I and three sub- 
divisions of encephalomere II appear. The latter remains, however, 
dorsally a simple expansion. Nerve relation of encephalomeres II and 
VII with somites 1 and 3 shown. 
Fig. 48. A frontal section in the dorsal part of encephalomeres I and II of an em- 
bryo with 20 to 82 somites, magnified 56 diameters. Only two vesicula- 
tions appear. Relation of thalamic portion of the trigeminus Anlage 
to these shown. 
Fig. 49. Frontal section in the dorsal portion of encephalomeres I and IT in an 
embryo with 19 or 20 somites. Two vesiculations only appear. 
Fig. 50. A frontal section of the same embryo as that shown in Figure 48, but 
more ventral, showing the constriction in the ventral wall of the mid- 
brain. 
Fig. 51. A frontal section of an embryo with 65 somites in the dorsal part of 
encephalomeres Land II (forebrain and midbrain), showing subdivision 
(thalamic) of the former. 
Fig. 52. A more ventral frontal section in the same series, showing a constriction 
in the lateral wall of the midbrain corresponding in position with the 
posterior commissure (coms. p.). The constriction in the forebrain 
corresponds with the superior commissure. 
Fig. 53. A frontal section, showing forebrain and midbrain regions in an embryo 
of 22 mm., magnified 28 diameters. Two constrictions only appear, 
one in the region of the superior commissure, and the other in the 
region of the posterior commissure. 
Fig. 54. A cross section of an embryo with 6 or 7 somites in the region of the 
cephalic plate, showing the ventral flexure of its edges. 
Fig. 55. A cross section of an embryo at a stage when the edges of the neural 
plate are about to be raised, showing the differentiation of a neural 
crest in the anterior part of the cephalic plate. Magnified 100 di- 
ameters. 
Fig. 56. A cross section in the posterior part of the cephalic plate of an embryo 
with 9 or 10 somites, showing that migration of neural-crest cells has 
already begun. Magnificd 85 diameters. 
