230 MR HAROLD AXEL HAIG ON 



Upon the ventral aspect, at the base of the brain, the infundibulum forms a down- 

 ward projection from the floor of the 3rd ventricle (PI. XX. fig. 2 3 ), whilst half way 

 between the infundibulum and a projection which marks the position of the ventral 

 part of the pons the 3rd nerve forms a noticeable feature (PI. XX. fig. 2 4 ). 



Additional features to be made out in a sagittal section taken through a cerebral 

 hemisphere, the optic thalamus and mid- and hind-brain (PL XX. fig. 4) : — In such 

 a section the general relations of the various regions of the brain may be studied to 

 a certain extent, and, moreover, by staining with hsematoxylin and eosin the main 

 distribution of grey and white matter at this stage of development may be made out. 

 The cortex cerebri (PI. XX. fig. 4 4 ' °) is made up of two parts, viz. : — 



(i.) A superficial layer of white fibres and neuroglia, forming a very narrow zone 



(PI. XX. fig. 4 5 ). 

 (ii.) A deeper layer of deeply-staining nerve-cells, with neuroglia, of somewhat 



wider extent than (i.). 



Deep to the cortex comes the white matter of the hemisphere, which is at this 

 stage not very thick. A section taken so as to pass completely through the cavity of 

 the prosencephalon shows in addition an inner layer, somewhat deeply stained, 

 which later on will form the ependymal lining of the lateral ventricle. 



Below the main mass of the hemisphere, part of the descending horn of the lateral 

 ventricle is seen (PI. XX. fig. 4"), and this becomes continuous anteriorly with 

 the rhinencephalon, passing into the olfactory lobe. The anterior boundary of the 

 descending horn is formed by a layer of grey matter, which above takes the form of 

 radiating streaks, alternating with white matter. 



In the region of the optic thalamus, small isolated patches of grey substance 

 appear near the dorsal surface, but in the region of the corpora quadrigemina white 

 matter seems as yet to predominate. 



The grey matter of the cerebellar hemisphere appears to lie chiefly on the surface, 

 but there is a deeper zone of small nerve-cells, the two layers being separated by a 

 clear zone of white matter : the cells of Purkinje do not as yet appear to have become 

 differentiated as a distinct line of neuroblasts. In the region of the medulla oblongata, 

 the nuclei of the 10th and 12th cranial nerves form a series of groups of rather large 

 nerve-cells (PI. XX. fig. 4 1 ) ; whilst the pyramidal tract appears as a well-defined 

 longitudinal set of fibres. 



The infundibulum, with a small portion of the cavity of the 3rd ventricle, has been 

 already noted ; the ventral mass of the pons is a very obvious feature lying just 

 anterior to the medulla (PI. XX. fig. 4). 



A fold of pia mater (PI. XX. fig. 4 3 ) shows between the overhanging posterior 

 extremity of the cerebral hemisphere and the mesencephalon, whilst in the deep 

 fissure anterior to the optic thalamus a small piece of choroid plexus appears. 



Points for comparison with the brains of other carnivora are the following : — 



