BRAIN OF THE ZBUGLODONTID^. 643 



factors, whatever their nature, played an enhanced r61e in the 

 ancestors of Cetacea, whereas they were recessive and negligible 

 in the Zeuglodontidfe. 



The evidence afforded by this cast therefore supports the 

 conclusions which we drew in tlie earlier part of this paper 

 concerning the deflection of the Zeuglodont line from the course 

 of true Cetacean evolution, but supports the conception that 

 iSqucdodon has at least close allinities with the true Cetacean 

 stock. 



8. The Trigeminus and the Law of Infiltration in 

 Cerebellar Evolution. 



In conclusion, it is necessary to call to mind certain general 

 facts about the cerebellum which appear to throw light upon the 

 strange and generalised hypertrophy of that organ in this group. 



We know that animals with very sensitive whiskers and 

 bristles (such as certain Rodentia) depend largely for their 

 power of equilibration upon stimuli which arise in nerve-endings 

 situated in relation with the proximal end of these whisker-like 

 structures and affected by their faintest movement. Such stimuli 

 are communicated to the cerebellum by way of the trigeminus. 

 In Ornithorhynchus, instead of bristles, there is a remarkable 

 development of special receptive end organs in the delicate snout 

 served by the trigeminus. The bristles round the mouth-parts 

 of the Sirenia may be active in a similar way in addition to their 

 function as tactile end organs. 



The more recent researches of Ingvar (1918) have placed the 

 morphological survey of the cerebellum by Elliot Smith upon an 

 even firmer basis : for he has extended liis research into the 

 lleptilian and Avian series and has shown that here, too, 

 the same sulci and three lobes are distinguishable. Such a 

 division into three lobes does not appear to obtain in Fishes or 

 Amphibia. It seems therefore that the advent of the middle 

 lobe synchronises with the origin of a neopallium in the cerebral 

 cortex. The researches of HaHcr(1900), of linger (1906), and 

 of Crosby (1917) have shown that the first clearly defined 

 neopallium pi-imordium makes its appearance in the Reptilia, and 

 in these creatures the tripartite structure of the cerebellum is 

 also clearly to be recognised for the first time. 



Recognising the relationship which the development of the 

 neopallium has to the expansion of the cerebellum in Mammalia 

 it is, at first, most disconcerting to find in a group of Eocene 

 mammals with such ill-developed neopallium a cerebellum of 

 extravagant proportions which has not merely obliterated all 

 ti-aces of the mid-brain from the surface but threatens to cover 

 the cerebrum also by its forward expansion. 



The cerebellum is usually relatively large in Eocene mammals 

 and yet such a bizarre arrangement as is present in Zeuglodonts 

 can have only one explanation. We may eliminate the neopallium 



Pboc. Zool. See— 1923, No. XLII. 42 



