The uppermost end of this does not till near the close of stage K become markedly constricted off from the remainder, but is nevertheless the rudiment of the pituitary body. Pl. 15, figs. 9a and 12, m shew in a most conclusive manner the correctness of the above account, and demonstrate that it is from the mouth involution, and not, as has usually been stated, from the alimentary canal, that the pituitary body is derived.
This fact was mentioned in my preliminary account of Elasmobranch development[276]; and has also been shewn to be the case in Amphibians by Götte[277]; and in Birds by Mihalkowics[278]. The fact is of considerable importance with reference to speculations as to the meaning of this body.
Plate 15, fig. 7 represents a transverse section through the head during a stage between I and K; but, owing to the cranial flexure, it cuts the fore part of the head longitudinally and horizontally, and passes through both the fore-brain (fb) and the hind-brain (iv.v.). Close to the base of the fore-brain are seen the mouth (m), and the pituitary involution from this (pt.). In contact with the pituitary involution is the blind anterior termination of the throat, which a little way back opens to the exterior by the first visceral cleft (I. v.c.). This figure alone suffices to demonstrate the correctness of the above account of the pituitary body; but the truth of this is still further confirmed by other figures on the same plate (figs. 9a and 12, m); in which the mouth involution is in contact with, but still separated from, the front end of the alimentary tract. By the close of stage K, the septum between the mouth and throat becomes pierced, and the two are placed in communication. This condition is shewn in Pl. 15, fig. 16a, and Pl. 16, figs. 1a, 1c, pt. In these figures the pituitary involution has become very partially constricted off from the mouth involution, though still in direct communication with it. In later stages the pituitary involution becomes longer and dilated terminally, while the passage connecting it with the mouth becomes narrower and narrower, and is finally reduced to a solid cord, which in its turn disappears. The remaining vesicle then becomes divided into lobes, and connects itself closely with the infundibulum (Pl. 16, figs. 5 and 6 pt). The later stages for Elasmobranchii are fully described by W. Müller in his important memoir on the Comparative Anatomy and development of this organ[279].
Development of the Cranial Nerves.
The present section deals with the whole development (so far as I have succeeded in elucidating it) of the cranial nerves (excluding the optic and olfactory nerves and the nerves of the eye-muscles) from their first appearance to their attainment of the adult condition. My description commences with the first development of the nerves, to this succeeds a short description of the nerves in the adult Scyllium, and the section is completed by an account of the gradual steps by which the adult condition is attained.
Early Development of the Cranial Nerves.—Before the close of stage H the more important of the cranial nerves make their appearance. The fifth and the seventh are the first to be formed. The fifth arises by stage G (Pl. 15, fig. 3, V), near the anterior end of the hind-brain, as an outgrowth from the extreme dorsal summit of the brain, in identically the same way as the dorsal root of a spinal nerve.
The roots of the two sides sprout out from the summit of the brain, in contact with each other, and grow ventralwards, one on each side of the brain, in close contact with its walls. I have failed to detect more than one root for the two embryonic branches of the fifth (ophthalmic and mandibular), and no trace of an anterior or ventral root has been met with in any of my sections.
The seventh nerve is formed nearly simultaneously with or shortly after the fifth, and some little distance behind and independently of it, opposite the anterior end of the thickening of the epiblast to form the auditory involution. It arises precisely like the fifth, from the extreme dorsal summit of the neural axis (Pl. 15, fig. 4a, VII). So far as I have been able to determine, the auditory nerve and the seventh proper possess only a single root common to the two. There is no anterior root for the seventh any more than for the fifth.
Behind the auditory involution, at a stage subsequent to that in which the fifth and seventh nerves appear, there arise a series of roots from the dorsal summit of the hind-brain, which form the rudiments of the glosso-pharyngeal and vagus nerves. These roots are formed towards the close of stage H, but are still quite short at the beginning of stage I. Their manner of development resembles that of the previously described cranial nerves. The central ends of the roots of the opposite sides are at first in contact with each other, and there is nothing to distinguish the roots of the glosso-pharyngeal and of the vagus nerves from the dorsal roots of spinal nerves. Like the dorsal roots of the spinal nerves, they appear as a series of ventral prolongations of a continuous outgrowth from the brain, which outgrowth is moreover continuous with that for the spinal nerves[280]. The outgrowth of the vagus and glosso-pharyngeal nerves is not continuous with that of the seventh nerve. This is shewn by Pl. 15, figs. 4a and 4b. The outgrowth of the seventh nerve though present in 4a is completely absent in 4b which represents a section just behind 4a.
Thus, by the end of stage I, there have appeared the rudiments of the 5th, 7th, 8th, 9th and 10th cranial nerves, all of which spring from the hind-brain. These nerves all develop precisely as do the posterior roots of the spinal nerves, and it is a remarkable fact that hitherto I have failed to find a trace in the brain of a root of any cranial nerve arising from the ventral corner of the brain as do the anterior roots of the spinal nerves[281].