Neal. — Nervous Syst. Squalus. 



PLATE 6. 



All Figures, except 40, magnified 100 diameters. Only the right half of the em- 

 bryo is shown in Figures 36-39, 42, and 44. 



Fig. 36. Frontal section of an embryo with 28 to 30 somites, showing the structure 

 of the neuromeres IV and V in the region of the " Deckplatte." 



Fig. 37. A more ventral frontal section in the same series cut in the region of the 

 lateral zones. The neuromeres appear as well marked local thickenings. 

 The radial arrangement of nuclei much less clearly shown than in the 

 preceding section (Fig. 3G). 



Fig. 38. A still more ventral section of the same series, in the region of the 

 " Grundplatte." The inner concavity appears as in the dorsal section 

 (Fig. 36). 



Fig. 39. Frontal section of an embryo with 28 to 30 somites, in the region of the 

 trunk, showing the structure of the myelomeres and their relation to 

 the somites. 



Fig. 40. A reconstruction from sections and dissected specimens of the anterior 

 end of an embryo with 28 to 30 somites, magnified 56 diameters. The 

 lumen of the neural tube is exposed so as to show the hindbrain neuro- 

 mere as local tliickenings of the left wall. Van Wijlie's somites, at 

 this stage separated by clearly marked constrictions, and Piatt's an- 

 terior somite, are seen. Cells, iu chief part derived from the neural 

 crest, are seen surrounding the mesodermic epithelium of the 1st and 

 2d visceral arches. 



Fig. 41. A cross section of an embryo with 28 to 30 somites in the trunk region. It 

 is seen that the somites press against the ventral half of the neural tube. 

 A migration of mesenchymatous cells from the sclerotome portion of 

 the somite has already begun. 



Fig. 42. Frontal section of an embryo with 50 somites (8 mm.) in the trunk region 

 (ectoderm omitted), taken in the region of the points of exit of the ven- 

 tral nerves. No constrictions in the ventral wall of the neural tube are 

 to be seen at this stage, but the ventral nerves lie opposite the middle 

 of the somites. 



Fig. 43. A more dorsal frontal section from the same series as Figure 42. Con- 

 strictions in the lateral wall, opposite which the ganglia lie, show no 

 corresponding ridges on the inner surface of the lateral wall. 



