ii HEMISPHEEES 91 



• Cerebral Hemispheres. — The hemispheres arise as bulgings of 

 the side wall of the fore-brain. As development goes on they increase 

 in size and grow first dorsalwards and later on forwards until in the 

 adult they are relatively very large. This increase in size is associ- 

 ated with a corresponding growth in the thickness of the wall of the 

 hemisphere — except at its hinder end next the thalamencephalon. 

 Here the inner wall of the hemisphere facing the thalamencephalon 

 remains relatively thin. 



About stage 35 a small rounded portion of this thin part of the 

 hemisphere wall bulges into its cavity — the lateral ventricle, This 

 ingrowth contains a vascular loop and is the rudiment of the choroid 

 plexus of the hemisphere or lateral plexus. The plexus grows rapidly 

 into the ventricular cavity, forming an irregular crumpled lamina 

 which in the adult attains to great size and complexity traversing 

 the whole lateral ventricle (Fig. 53, F and G, l.p). No doubt this, 

 by diffusion between the blood in its vessels and the fluid in the 

 lateral ventricle, helps to provide for the nutritive and respiratory 

 needs of the hemisphere wall. 



During the later development of the hemisphere its walls become 

 differentiated into regions in the manner described by Elliot Smith 

 (1908). Most important from the point of view of general vertebrate 

 morphology is the fact that a distinct cortex is developed in the 

 form of a layer of ganglion-cells traversing the roof of the hemisphere 

 parallel to its surface, and at about one-third of the distance from 

 the surface to the ventricular cavity. This cortex extends on the 

 one hand just on to the mesial face of the hemisphere and on the 

 other to a point rather more than one-third of the distance from 

 dorsal to ventral edge on the outer face of the hemisphere. 



Of this cortical formation, which constitutes the archipallium, 

 the mesial portion corresponds to the hippocampus of higher verte- 

 brates, and the outer portion to the pyriform lobe. The neo- 

 pallium which in the higher forms becomes interposed between these 

 does not appear yet to have become distinctly recognizable in 

 Zepidosiren. 



Less important from the point of view of general morphology 

 but more conspicuous in their structural expression are certain 

 changes which take place in relation to the olfactory apparatus. 



The portion of hemisphere wall to which the first cranial nerve is 

 attached — the olfactory bulb — is at first simply part of the lateral 

 wall of the hemisphere but as development proceeds it is found to 

 take the form of a sort of cap lying on the dorsal side or roof of the 

 hemisphere at about the middle of its length as viewed from above. 

 This change in position is brought about by an enormous hyper- 

 trophy of the portion of the ventral wall of the hemisphere which 

 lies in front of the optic chiasma — the olfactory tubercle. 



Later on, from stage 38, the portion of hemisphere roof lying 

 posterior to the olfactory bulb undergoes active growth in length 

 with the result that the bulb is gradually carried forwards and 



