XII.] HISTOGENESIS. 385 



shallow 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 KeU. 



The fissures of the cerebrum may be divided into two classes ; 

 (1) the primitive, (2) the secondary fissures. The primitive fissures 

 are the first to appear ; they owe their origin to a folding of the 

 entire wall of the cerebral vesicles. Many of them are transient 

 structures and early disappear. The most important of those 

 which persist are the hippocampal, the parieto-oocipital, the 

 calcarine (in Man and Apes) sulci and the Sylvian fissures. 

 The secondary fissures appear later, and are due to folds which 

 implicate the cortex of the hemispheres only. 



The olfactory lobes. The olfactory lobes, or rhinen- 

 cephala, are secondary outgrowths of the cerebral hemi- 

 spheres, and contain prolongations of the lateral ven- 

 tricles, which may however be closed in the adult state ; 

 they arise at a fairly early stage of development from 

 the under and anterior part of the hemispheres (Fig. 

 127). 



Histogenetic changes. The walls of the brain are 

 at first very thin and, like those of the spinal cord, are 

 formed of a number of ranges of spiadle-shaped ceUs. 

 In the floor of the hind- and mid-brain a superficial 

 layer of delicate nerve-fibres is formed at an early 

 period. This layer appears at first on the floor and 

 sides of the hind-brain, and almost immediately after- 

 wards on the floor and the sides of the mid-brain. 

 The cells internal to the nerve-fibres become differen- 

 tiated into an innermost epithelial layer lining the 

 cavities of the ventricles, and an outer layer of grey 

 matter. 



The similarity of the primitive arrangement and 

 F. & B. 25 



