Part 12

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 (1pp, 2pp). (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 (1pp).

Figs. 9a and 9b. Two longitudinal sections through the brain of a Pristiurus embryo belonging to a stage intermediate between I and K. Zeiss A, ocul. 1.

9a 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.

9b 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. 13a. 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. 13b. Transverse section through the last arch but one of the same embryo as 13a. 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. 15a and 15b. Two longitudinal and vertical sections through the head of a Pristiurus embryo belonging to stage K. Zeiss a, ocul. 3.

15a 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.

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

16a 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.

16b is a more superficial section shewing the head-cavities pp 1, 2, 3, and the lower vagus commissure.

PLATE 16. (X. p. 397.)

COMPLETE LIST OF REFERENCE LETTERS.

auv. Auditory vesicle. cb. Cerebellum. cer. Cerebral hemispheres. ch. Notochord. cin. Internal carotid. ft. Fasciculi teretes. in. Infundibulum. lv. Lateral ventricle. mb. Mid-brain, or optic lobes. md. Medulla oblongata. mn. Mandible. ol. Olfactory pit. oll. Olfactory lobe. op. Eye. opn. Optic nerve. opth. Optic thalamus. pc. Posterior commissure. pcl. Posterior clinoid. pn. Pineal gland. pt. Pituitary body. rt. Restiform tracts. tv. Tela vasculosa of the roof of the fourth ventricle. iv.v. Fourth ventricle. vii. Seventh nerve. x. Rudiment of septum which will grow backwards and divide the unpaired cerebral rudiment into the two hemispheres.

Figs. 1a, 1b, 1c. Longitudinal sections of the brain of a Scyllium embryo belonging to stage L. Zeiss a, ocul. 1.

1a is taken slightly to one side of the middle line, and shews the general features of the brain, and more especially the infundibulum (in) and pituitary body (pt).

1b is through the median line of the pineal gland.