154 



Mr. G. A. Watson. 



[July 28, 



more difficult than In the Mole, a similar large field of comparatively -well- 

 developed cortex can be distinguished on the dorso-lateral and mesial 

 aspects, occupying the same relative position and being of about the same 

 relative size, as are Areas I and II taken together in the Mole. No satis- 

 factory criteria, however, have been found from cell lamination alone by 

 the aid of which this region can be divided into Areas I and II as in the 

 Mole. Hence, it is considered that although there may be some attempt 

 at differentiation of these areas in the Hedgehog, it is probably more 

 correct to regard the entire field in the latter as a combined sensori-motor area. 



The explanation which suggests itself of these structural differences is that 

 they stand in relation to the fact that the Mole and Shrew are possessed of 

 more numerous and better motor accomplishments than the Hedgehog, and 

 so the former have a more specialised zone of " motor " cortex, the latter 

 being a comparatively lethargic animal, and dependent for its survival rather 

 upon its protective armour of spines than upon its activity. 



Areas of Undifferentiated Cortex. — In the anterior (extending also to the 

 mesial), lateral, and posterior portions of the neopallium are three fields of 

 moderate size in all three animals, to which, from the want of any special 

 feature in their lamination, no attempt is made at the present time to 

 attach any peculiar functional value. They ai'e regarded as indifferent or 

 unspecialised regions of cortex. 



The area of the cortical distribution of the eighth nerve has not been 

 definitely localised. By analogy one would expect to find this probably in 

 about the middle and towards the anterior part of the outer portions of the 

 field termed Area II in the Mole, or just external to this, and in the 

 corresponding region in the Hedgehog. 



2. Anterior Surface. — Occupying almost the whole of the anterior and 

 inferior aspect of the frontal pole and curving forwards to join the olfactory 

 bulb, is a small area of comparatively well-developed cortex, sharply marked 

 off from the less differentiated cortex posteriorly. This is of about the same 

 relative size in the Mole and Hedgehog, and probably constitutes a neopallial 

 representation of the olfactory sense. 



3. Mesial Surface. — This may be divided into (1) the straight anterior 

 portion, and (2) the curved postero-ventral portion which arches over and 

 round the mesencephalon and hippocampus (figs. 2 and 3). 



(1) The straight portion presents anteriorly an area of undifferentiated 

 cortex continuous with that on the dorsal aspect, posteriorly a field with 

 modified motor characters joining that in the dorsal surface, and between 

 these an area with some indefinite sensory characters. Similar areas are 

 seen in the Hedgehog. 



