In most Vertebrates the olfactory lobes arise at a fairly early stage of development from the under and anterior part of the hemispheres ([fig. 250] olf). In Elasmobranchs they arise, not from the base, but from the lateral parts of the brain ([fig. 263]), and become subsequently divided into a bulbous portion and a stalk. They vary considerably in their structure in the adult.
Fig. 263. Section through the brain and olfactory organ of an embryo of Scyllium. (Modified from figures by Marshall and myself.)
ch. 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. anterior prolongation of pineal gland.
In Amphibia the solid anterior prolongations of the cerebral hemispheres already spoken of are usually regarded as the olfactory lobes, but according to Götte, whose view appears to me well founded, small papillæ, situated at the base of these prolongations, from which olfactory nerves spring, and which contain a process of the lateral ventricle, should properly be regarded as the olfactory lobes. These papillæ arise prior to the solid anterior prolongations of the hemispheres.
In Birds the olfactory lobes are small. In the chick they arise (Marshall) on the seventh day of incubation.
General conclusions as to the Central Nervous System.
It has been shewn above that both the brain and spinal cord are primitively composed of a uniform wall of epithelial cells, and that the first differentiation results in the formation of an external layer of white matter, a middle layer of grey matter (ganglion cells), and an inner epithelial layer. This primitive histological arrangement, which in many parts of the brain at any rate, is only to be observed in the early developmental stages, has a simple phylogenetic explanation.
As has been already explained in an earlier part of this chapter the central nervous system was originally a differentiated part of the superficial epidermis.
This differentiation (as may be concluded from the character of the nervous system in the Cœlenterata and Echinodermata) consisted in the conversion of the inner ends of the epithelial cells into nerve-fibres; that is to say, that the first differentiation resulted in the formation of a layer of white matter on the inner side of the epidermis. The next stage was the separation of a deeper layer of the epidermis as a layer of ganglion cells from the superficial epithelial layer, i.e. the formation of a middle layer of ganglion cells and an outer epithelial layer. Thus, phylogenetically, the same three layers as those which first make their appearance in the ontogeny of the vertebrate nervous system became successively differentiated, and in both cases they are clearly placed in the same positions, because the central canal of the vertebrate nervous system, as formed by an involution, is at the true outer surface, and the external part of the cord is at the true inner surface.