Fig. 4 a and 4 h. Two sections through the head of a Pristiurus embryo 

 of stage I. They shew (1) the appearance of the seventh nerve. (2) The 

 portion of the body cavity belonging to the first and second visceral arches. 

 (3) The commencing thickening of epiblast to form the auditory involution. 



In 4 1), the posterior of the two sections, no trace of an auditory nerve is 

 to be seen. 



Fig. 5 a and 5&. Two sections through the head of a Torpedo embryo 

 with 3 visceral clefts. Zeiss A, ocul. 1. 



5 a shews the formation of the thin roof of the fourth ventricle by a 

 divarication of the two lateral halves of the brain. 



Both sections shew tlie commencing formation of the thyroid body {th) 

 at the base of the mandibular arch. 



They also illustrate the formation of the visceral clefts by an outgrowth 

 from the alimentary tract without any corresponding ingrowth of the external 

 epiblast. 



Fig. 6. Section through the hind-brain of a somewhat older Torpedo 

 embryo. Zeiss A, ocul. 1. 



The section shews (Ij the attachment of a branch of the vagus to the 

 walls of the hind-brain. (2) The peculiar form of the hind-brain. 



Fig. 7. Transverse section through the head of a Pristiurus embryo 

 belonging to a stage intermediate between I and K, passing through both 

 the fore-brain and the hind-brain. Zeiss A, ocul. 1. 



The section illustrates (1) the formation of the pituitary body {pt) from 

 the mouth involution (m), and proves that, although the wall of the throat 

 {al) is in contact with the mouth involution, there is by this stage no 

 communication between the two. (2) The eye. (3) The sections of the 

 body cavity in the head, I pp. 1pp. (4) The fifth nerve (v.) and the seventh 

 nerve (vii.). 



Fig. 8. Transverse section through the brain of a rather older embryo 

 than fig. 7. It shews the ventral junction of the anterior sections of the 

 body cavity in the head, 1 pp. 



Fig. 9« and 9&. Two longitudinal sections through the brain of a 

 Pristiurus embryo belonging to a stage intermediate between I and K. 

 (Zeiss A, ocul. 1.) 



Fig. 9 a is taken through the median line, but is reconstructed from 

 two sections. It shews (1) The divisions of the brain — The cerebrum and 

 thalamcncephalon in the fore-brain ; the mid-brain ; the commencing 

 cerebellum in the hind-brain. (2) The relation of the mouth involution 

 to the infundibulum. (3) The termination of the notochord. 



Fig. 96 is a section to one side of the same brain. It shews (1) The 

 divisions of the brain. (2) The point of outgrowth of the optic nerves, op. n. 

 (3) The sections of the body cavity in the head and the bifurcation of the 

 optic nerve over the 2nd of these. 



Fig. 10. Longitudinal section through the head of a Pristiurus embryo 

 somewhat younger than fig. 9. Zeiss a, ocul. 4. It shews the relation of 

 the nerves and the junction of the fifth, seventh, and auditory nerves with 

 the brain. 



Fig. 11. Longitudinal section through the fore-brain of a Pristiurus 

 embryo of stage K, slightly to one side of the middle line. It shews the 

 deep constriction separating the thalamcncephalon from the cerebral hemi- 

 spheres. 



Fig. 12. Longitudinal section through the ba^c of the brain of an 

 embryo of a stage intermediate between I and K. 



