NEAL., — Nervous Syst. Squalus. 
Fig. 23. 
Fig. 24. 
Fig. 25. 
Fig. 26. 
Fig. 27. 
Fig. 28. 
Fig. 29. 
Fig. 30. 
Fig. 31. 
Fig. 32. 
Fig. 33. 
Fig. 34. 
Fig. 35. 
PLATE 5. 
All the Figures except 31 and 32 represent frontal sections of embryos viewed 
from the dorsal side. All except Figures 25, 32, and 35 are magnified 43 diameters. 
Fig. 22. 
A frontal section of an embryo with 14 or 15 somites. Eincephalomere IV 
appears as a thickening of the lateral walls of the neural tube. No 
local thickening seen in the region of encephalomere III. 
From an embryo with 16 or 17 somites. A local thickening of the lateral 
walls in the posterior part of encephalomere III appears. 
From an embryo with 19 or 20 somites. The first four hindbrain neuro- 
meres are now seen as local thickenings of the lateral walls, the 
thickening of neuromere III affecting its posterior part only. 
From an embryo with 28 to 30 somites, magnified 75 diameters. Five 
hindbrain neuromeres are seen. The auditory invagination appears 
opposite encephalomere VI. 
From an embryo with 50 somites (8 mm.) in the region of the “ Deck- 
platte,” showing the faintly marked expansions of the encephalomeres. 
A more ventral section of the same embryo. The encephalomeres sharply 
defined by constrictions. A ‘secondary constriction in encephalomere 
III appears. 
A more ventral section of the same series, in the region of the lateral 
zones. The local thickenings of the encephalomeres well marked. 
A still more ventral section of the same embryo. ‘The inner cusps be- 
tween the neuromeres appear as in the more dorsal sections (Fig. 27). 
Frontal section in the region of the lateral zones of an embryo of 15 mm. 
The structure of the neuromeres is seen to be the same as that 
described by Orr (’87) for the Lizard. 
Cross section of an embryo with 20 somites, in the region of encephalo- 
mere IV, to show the thickening of the lateral zones. 
Cross section of an embryo with 28 to 80 somites in the posterior region 
of encephalomere III. The lateral zones more markedly thickened 
than in the previous stage (Fig. 31). 
Frontal section of an embryo with 50 somites (8 mm.), killed with a mix- 
ture of chromic, picric, and sulphuric acids, showing great intensifica- 
tion of the neuromeres, as the result of contraction due to inadequate 
fixation. The drawing, however, exaggerates the phenomena, since it 
represents the nuclear regions of the medullary wall with deeper 
shading. 
Frontal section of a 19-day Swine embryo. The constrictions between 
the neuromeres are sharply defined. 
Frontal section of an embryo of Amblystoma shortly after the closure of 
the neural tube. The neural tube is sharply outpocketed in the regions 
of proliferation of the ganglionic Anlagen of nerves V and VIT. No 
evidence of a thickening or outpocketing comparable with encepha- 
lomere IV appears either at this or later stages. 
