366 



THE OLFACTORY LOBES. 



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



root of the hemispheres 

 — the hilus of the bean 



.i^ .^^'v-T^aa^^f'-^ ^^ — there is formed a shal- 



<?A>' ^ .^^^^^s^^^Mm^^ ^^^ \^^y^ 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- 

 dai"y outgrowths of the 

 cerebral hemispheres, and 

 contain prolongations of 

 the lateral ventricles, 

 but may however be 

 solid in the adult state. 

 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. 26*3), and become 

 subsequently divided into a bulboiis 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 papilla 

 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 (\B.y of incubation. 



Fig. 263. Section thkough the brain and ol- 

 factoky organ of an embryo of scyllium. (modi- 

 fied from figures by Marshall and myself.) 



cli. cerebral hemispheres; ol.v. olfactory vesicle; 

 olf. olfactory pit; Sch. Schneiderian folds; /. olfac- 

 tory nerve. The reference line has been accidentally 

 taken through the nerve to the brain; pn. anterior 

 prolongation of pineal gland. 



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, 



