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 thalamencephalon 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. 9_b_ 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 (_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. 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 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 (1) the condition of the end of the notochord; (2) the relation of the mouth involution to the infundibulum. Fig. 13_a_. Longitudinal and horizontal section through part of the head of a Pristiurus 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 (_opn_) 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_b_. Transverse section through the last arch but one of the same embryo as 13_a_. 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. Figs. 15_a_ and 15_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.