366 



THE OLFACTORY LOBE*. 



73 n 



corpora striata, have assumed a somewhat beau-shaped form. At the 



root of the hemispheres 

 the hilus of the bean 

 there is formed a shal- 

 low depression, which 

 constitutes the first trace 

 of the Sylvian fissure. 

 The part of the brain 

 lying in this fissure is 

 known as the island of 

 Reil. 



The olfactory lobes. 

 The olfactory lobes, or 

 rhinencephala, are secon- 

 dary outgrowths of the 

 FIG. 263. SECTION THROUGH TIIK BEAIN AN,, OL- eere bral hemispheres, and 



FACTOKY ORGAN OF AN EMBRYO OF ScYLLU'M. (Modi- . 



lied from figures by Marshall and myself.) Contain prolongations ot 



(/>. cerebral hemispheres; o/.r. olfactory vesicle; the lateral ventlicles, 

 o/f. olfactory pit; Kch. Schneiderian folds; 7. olfac- but, may however be 

 to'ry nerve. ' The reference line has been accidentally 1-1 ,, n( ] n U ofitp 

 taken through the nerve to the brain; pn. anterior Solld in tue adult State 

 prolongation of pineal gland. According to Marshall 



they develop in Birds 



and Elasmobranchs and presumably other forms later than the ol- 

 factory nerves, so that the olfactory region of the hemispheres is 

 indicated before the appearance of the olfactory lobes. 



In most Vertebrates the olfactory lobes arise at a fairly early 

 stage of development from the under and anterior part of the hemi- 

 spheres (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. 



In Amphibia the solid anterior prolongations of the cerebral hemi- 

 spheres already spoken of are usually regarded as the olfactory lobes, 

 but according to Gotte, whose view appears to me well founded, small 

 papillae, situated at the base of these prolongations, from which olfac- 

 tory nerves spring, and which contain a process of the lateral ventricle, 

 should properly be regarded as the olfactory lobes. These papillse 

 arise prior to the solid anterior prolongations of the hemispheres. 



In Birds the olfactory lob< j s 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, 



