It shews (1) The condition of the end of the notochord. (2) The relation 

 of mouth invokition to the infimdibulum. 



Fig. 13 a. Longitudinal and horizontal section through part of the head 

 of a Pristiunis embryo rather older than K, Zeiss A, ocul. 1. 



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

 Thus the optic nerve {op. 7i) 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. 13 h. Transverse section through the last arch but one of the same 

 embryo as \Za. Zeiss A, ocul. 1. 



The figure shews (1) The mode of formation of a visceral cleft without 

 any involution 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. 13, 

 considerably 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. 



Fig. Id a and 15 &. Two longitudinal and vertical sections through 

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



Fig. 15 a is the most superficial section of the two. It shews the con- 

 stitution 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. 



Fig. 15 b mainly illustrates the dorsal commissure of the vagus nerve {com) 

 continuous ^vith the dorsal commissures of the posterior root of the spinal 

 nerves. 



Fig. 16. Two longitudinal and vertical sections of the head of a 

 Pristiurus embryo belonging to the end of stage K. Zeiss a, ocul. 1. 



Fig. 16 « passes through the median line of the brain and shews the 

 infundibiilum, 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. 



Fig. 16 & is a more superficial section shelving the head cavities p)P' Ij 2, 

 3, and the lower vagus commissm-e. 



