886 DEVELOPMENT OF ORGANS IN MAMMALIA. [CHAP. 



Section through the Brain and Olfactory Organ of an 

 Embryo of Scyllium. 



ch. cerebral hemispheres ; ol.v. olfaotory vesicle ; olf. olfactory 

 pit ; Sch. Schneiderian folds ; 1. olfactory nerve (the reference 

 line has been accidentally carried through the nerve so as to 

 appear to indicate the brain) ; pn. anterior prolongation of 

 pineal gland. 



histological characters of the parts of the brain behind 

 the cerebral hemispheres to those of the spinal cord is 

 very conclusively shewn by the examination of any good 

 series of sections. In both brain and spinal cord the 

 white matter forms a cap on the ventral and lateral 

 parts some considerable time before it extends to the 

 dorsal surface. In the medulla oblongata the white 

 matter does not eventually extend to the roof owing to 

 the peculiar degeneration which that part undergoes. 



In the case of the fore-brain the walls of the hemi- 

 spheres become first divided (KoUiker) into a superficial 

 thinner layer of rounded elements, and a deeper and 

 thicker epithelial layer, and between these the fibres of 



