NERVOUS SYSTEM OF THE VERTEBRATA. 



445 



stage of development from the under and anterior part of the 

 hemispheres (fig. 250 olf}. In Elasmobranchs they arise, not 



/O '.71 



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. Schnei- 

 derian folds ; 1. olfactory nerve. The reference line has been accidentally taken through 

 the nerve to the brain; pn. anterior prolongation of pineal gland. 



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 

 hemispheres 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 olfactory nerves spring, and which 

 contain a process of the lateral ventricle, should properly be 

 regarded as the olfactory lobes. These papillae 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 tlte 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 



