156 



Mr. G. A. Watson. 



[July 28, 



larger in the Mole than in the Hedgehog, but in both, comparatively to 

 Divisions 1 and 3, it presents unspecialised features. The inferior portion 

 (Division 3) is relatively not .only considerably larger in the Hedgehog than 

 in the Mole, but in the former it is a more histologically distinct field, 

 showing not only a well-formed " granular " layer (the cellular elements 

 in which, however, are mostly angular or small pyramidal in shape), but 

 a comparatively good infra-granular and a definite, though shallow, supra- 

 granular layer. In the Shrew this division is rather better developed than 

 in the Mole. 



Owing to the presence throughout this region of such a deep and definite 

 granular layer, it is concluded that this field is sensory in function, and 

 regarding it the following suggestions are made : — 



(a) The area is much too large to be concerned only with the cortical 

 distribution of the optic nerves, which are relatively so minute in these 

 animals, especially in the Mole. 



(6) The inferior portion (Division 3) only is visual. In the Mole this 

 portion is a mere vestige ; in the Hedgehog it is better developed and 

 relatively larger, having apparently extended upwards somewhat and 

 encroached upon the area of unspecialised granular cortex (Division 2) as 

 compared with the Mole. 



(c) The middle and superior portions (Divisions 2 and 1) may correspond 

 to the large infra-calcarine area of certain relatively higher mammals (e.g., 

 Ungulata and Carnivora), in which, owing to the greater development of the 

 visual faculty, the inferior portion (Division 3, visual) has, so to speak, 

 extended upwards, backwards, outwards, and forwards so as to overlie the 

 middle and superior portions, and has become the calcarine region. 



(d) The relatively well-developed superior and anterior portion (Division 1) 

 of the two specialised divisions in both Mole and Hedgehog may be concerned 

 with the cortical distribution of the fifth sensory nerve. On account of the 

 importance of the fifth sensory nerve as an avenue of information to these 

 mammals through snout or vibrissas touch, or both, and, in view of the large 

 size in them of the fifth nerve, it seems probable that the sensory portion of 

 this nerve should have a very special cortical representation. 



The Cerebral Cortical Layers. (Neopallium.) 



Although the total depth of the cortex in the best developed regions is 

 different in the Mole, Shrew, and Hedgehog, the relative depth of the 

 separate -layers, supra- and infra- granular particularly, appears to be about 

 the same in all. The following micrometric measurements of the cortical 

 layers in three areas of the Mole's cortex, which have been kindly furnished 



