The Olfactory nerve. It was shewn in my monograph on Elasmobranch Fishes that the olfactory nerve grew out from the brain in the same manner as other nerves; and Marshall (No. [355]), to whom we are indebted for the greater part of our knowledge on the development of this nerve, has proved that it arises prior to the differentiation of the olfactory lobes.
The earliest stages in the development of the nerve have not been made out. Marshall, as already stated, finds that in the Chick the neural crest is continued in front of the optic vesicles, and holds that this fact is strong a priori evidence in favour of the nerve growing out from it. As mentioned above, note on p. [456], I cannot without further evidence accept Marshall’s statements on this point. In any case Marshall has not yet been able again to find an olfactory nerve till long after the disappearance of the neural crest. The olfactory nerve at the next stage observed forms an outgrowth of fusiform cells springing on either side from near the summit of the fore-brain; and at fifty hours it ends close to a slight thickening of the epiblast forming the first rudiment of the olfactory pit, with the walls of which it soon becomes united.
Fig. 273. Section through the brain and olfactory organ of an embryo of Scyllium. (Modified from figures by Marshall and myself.)
c.h. cerebral hemispheres; ol.v. olfactory vesicle; olf. olfactory pit; Sch. Schneiderian folds; I. olfactory nerve. The reference line has been accidentally taken through the nerve to the brain; pn. pineal gland.
The growth of the cerebral hemispheres causes its point of insertion in the brain to be relatively shifted; and on the development of the olfactory lobes (vide pp. [444], [445]) it arises from them ([fig. 273]). In Elasmobranchs there is a large development of ganglion cells near its root. From Marshall’s figures these appear also to be present in the Chick, but they do not seem to have been found in other forms. In both Teleostei and Amphibia the olfactory nerves are at first extremely short.
Marshall holds that the olfactory nerve is a segmental nerve equivalent to the third, fifth, seventh etc. nerves. It has been already stated that in my opinion the origin of the olfactory nerves from the fore-brain, which I hold to be the ganglion of the præoral lobe, negatives this view. The mere fact of these nerves originating as an outgrowth from the central nervous system is no argument in favour of Marshall’s view of their nature; and even if Marshall’s opinion that they arise from the neural crest should turn out to be well founded, this fact would not prove their segmental nature, because their origin from this crest would, as indicated in the next paragraph, merely seem to imply that they primitively arose from the lateral borders of the nerve-plate from which the cerebrospinal tube has been formed.
Situation of the dorsal roots of the cranial and spinal nerves. The probable explanation of the origin of nerves from the neural crest has already been briefly given (p. [316]). It is that the neural crest represents the original lateral borders of the nervous plate, and that, in the mechanical folding of the nervous plate to form the cerebrospinal canal, its two lateral borders have become approximated in the median dorsal line to form the neural crest. The subsequent shifting of the nerves I am unable to explain, and the meaning of the transient longitudinal commissure connecting the nerves is also unknown. The folding of the neural plate must have extended to the region of the origin of the olfactory nerves, so that, as just stated, there would be no special probability of the olfactory nerves belonging to the same category as the other dorsal nerves from the fact of their springing from the neural crest.
Bibliography of the Peripheral Nervous System.
(351) F. M. Balfour. “On the development of the spinal nerves in Elasmobranch Fishes.” Philosophical Transactions, Vol. CLXVI. 1876; vide also, A monograph on the development of Elasmobranch Fishes. London, 1878, pp. 191-216.
(352) W. His. “Ueb. d. Anfänge d. peripherischen Nervensystems.” Archiv f. Anat. u. Physiol., 1879.
(353) A. M. Marshall. “On the early stages of development of the nerves in Birds.” Journal of Anat. and Phys., Vol. XI. 1877.
(354) A. M. Marshall. “The development of the cranial nerves in the Chick.” Quart. J. of Micr. Science, Vol. XVIII. 1878.
(355) A. M. Marshall. “The morphology of the vertebrate olfactory organ.” Quart. J. of Micr. Science, Vol. XIX. 1879.
(356) A. M. Marshall. “On the head-cavities and associated nerves in Elasmobranchs.” Quart. J. of Micr. Science, Vol. XXI. 1881.
(357) C. Schwalbe. “Das Ganglion oculomotorii.” Jenaische Zeitschrift, Vol. XIII. 1879.