442 DEVELOPMENT OF ELASMOBRANCH FISHES. 



gb is a section to one side of the same brain. It shews (i) The divisions of the 

 brain. (2) The point of outgrowth of the optic nerves (opn). (3) The sections of 

 the body-cavity in the head and the bifurcation of the optic nerve over the second 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. ii. 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 thalamencephalon from the cerebral hemispheres. 



Fig. 12. Longitudinal section through the base of the brain of an embryo of a 

 stage intermediate between I and K. 



It shews (i) the condition of the end of the notochord ; (2) the relation of the 

 mouth involution to the infundibulum. 



Fig. i$a. Longitudinal and horizontal section through part of the head of a 

 Pristiurus embryo rather older than K. Zeiss A, ocul. i. 



The figure contains the eye cut through in the plane of the choroid slit. Thus the 

 optic nerve (op n) and choroid slit (ch) are both exhibited. Through the latter is 

 seen passing mesoblast accompanied by a blood-vessel (v). Op represents part of the 

 optic vesicle to one side of the choroid slit. 



No mesoblast can be seen passing round the outside of the optic cup ; and the only 

 mesoblast which enters the optic cup passes through the choroid slit. 



Fig. I 3^- Transverse section through the last arch but one of the same embryo 

 as 1 3 a. Zeiss A, ocul. i. 



The figure shews ( i ) The mode of formation of a visceral cleft without any involu- 

 tion of the external skin. (2) The head-cavity in the arch and its situation in relation 

 to the aortic arch. 



Fig. 14. Surface view of the nasal pit of an embryo of same age as fig. 1 3, con- 

 siderably magnified. The specimen was prepared by removing the nasal pit, flattening 

 it out and mounting in glycerine after treatment with chromic acid. It shews the 

 primitive arrangement of the Schneiderian folds. One side has been injured. 



Figs. 1 5 a and i$b. Two longitudinal and vertical sections through the head of a 

 Pristiurus embryo belonging to stage K. Zeiss a, ocul. 3. 



15 a is the most superficial section of the two. It shews the constitution of the 

 seventh and fifth nerves, and of the intestinal branch of the vagus. The anterior 

 branch of the seventh nerve" deserves a special notice. 



15 mainly illustrates the dorsal commissure of the vagus nerve (com) continuous 

 with the dorsal commissures of the posterior root of the spinal nerves. 



Fig. 1 6. Two longitudinal and vertical sections of the head of a Pristiurus 

 embryo belonging to the end of stage K. Zeiss a, ocul. i. 



16 a passes through the median line of the brain and shews the infundibulum, 

 notochord and pituitary body, etc. 



The pituitary body still opens into the mouth, though the septum between the 

 mouth and the throat is broken through. 



i6b is a more superficial section shewing the head-cavities // i, 2, 3, and the 

 lower vagus commissure. 



