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 tlie 4th and 5th. 



Fig. 46. A parasagittal section of a Squalus embryo with 28 to 30 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 niesodermic 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 enceplialomore II appear. The latter remains, however, 

 dorsally a simple expansion. Nerve relation of encephalomeres II and 

 VII witli somites 1 and 3 sliown. 



Fig. 48. A frontal section in the dorsal part of encephalomeres I and II of an em- 

 bryo with 30 to 32 somites, magnified 50 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 II 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 (35 somites in the dorsal part of 

 encephalomeres land II (forebrain and midbrain), showing subdivision 

 (tlialaniio) of the former. 



Fig. 52. A more ventral frontal section in the same series, showing a constriction 

 in the lateral wail of the midbrain corresponding in position with the 

 posterior commissure {corns, p.). Tiie 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 23 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 embrj'o witli G 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. Magnified 85 diameters. 



