94 EMBEYOLOGY OF THE LOWEE VEETEBEATES ch. 



result of the heavily yolked condition of the egg. As a result of the 

 concentration of yolk towards the ventral side in such heavily yolked 

 Vertebrates the processes of growth are retarded upon that side. But 

 it is clear that retardation of growth in length on the ventral side as 

 compared with the .dorsal would bring about a flexure towards the 

 ventral side. That the cerebral flexure is due rather to such a general 

 cause than to any inherent peculiarity in the brain itself is supported 

 by the fact that the notochord is also strongly flexed (see Fig. 54, C). 



As a consequence of these considerations, we are inclined to take 

 the view that the phenomenon of cerebral flexure is of much less 

 fundamental morphological significance than is commonly supposed. 



Comparing the later stages figured for Acanthias with those of 

 Lepidosiren, it will be seen that the brain shows the same elements 

 although these differ in their relative size and other features in the 

 two cases. Thus the cerebellum of the sharks — correlated with the 

 active and complex movements of these fishes — becomes much more 

 developed. It grows greatly in anteroposterior extent and that 

 causes it to bulge outwards as shown in Fig. 54, E (cer). 



The pineal body is slender and elongated in form : the velum 

 forms a conspicuous infolding of the thalamencephalic roof continuous 

 across the mesial plane. 



The wall of the anterior portion of the primitive fore-brain under- 

 goes a fairly uniform increase in thickness throughout with the excep- 

 tion of a transverse band just in front of the velum which becomes 

 thin and membranous. This portion of the brain increases somewhat 

 in transverse diameter so that it is broad in shape as seen from above, 

 but there is no definite bulging in its side wall to form a distinct 

 hemisphere. The material that would ordinarily go to form the 

 hemispheres remains here in the thickness of the wall. 



The olfactory bulb arises as a slight projection from the side wall 

 of the fore-brain, but as development proceeds and the olfactory organ 

 becomes removed from the brain by the interposition of mesenchyme 

 the olfactory bulb remains in contact with the olfactory organ, its 

 attachment to the brain becoming drawn out into a more or less 

 elongated stalk the olfactory peduncle or olfactory tract. 



The salient features in the establishment of the topography of the 

 Vertebrate brain have been illustrated in outline in the sketch which 

 has just been given. It would be beyond the scope of this work to 

 make any attempt to fill in the picture in detail but it is necessary to 

 recall a few points which are of interest to morphologists apart from 

 specialists in neurology. 



It should in the first place be borne clearly in mind that the 

 brain — like indeed the whole of the nervous system (see below, p. 118) 

 is to be looked upon as a fundamentally continuous structure. The 

 parts which compose the adult brain — medulla, cerebellum, mesen- 

 cephalon and so on — are not to be regarded as constituent units 

 which go to build up the complete brain, but rather as specialized 

 portions of a once homogeneous whole. The process of specialization 



