ZENTRALNERVENSYSTEM ZAHN- UND BARTENWALE 



1031 



view, therefore, it is not the cortex which détermines ultrasonic capacity, but other 

 substrata to be found in deeper levels of the brain. Thus for example the outer 

 segment of the upper olive and the ventral nucleus of the latéral lemniscus are very 

 highly developed in the dolphin, but so reduced in man that the capacity to perceive 

 ultrasonic waves has been lost. 



Our research on Platanista gangetica agrées very well with Zvorykin's views. 

 PlatanistcCs cortex is still only slightly differentiated. The subcortical acoustic 

 System, however, and especially the colliculi posteriores, show a relatively high 

 development. 



Zvorykin (1963) is of the opinion that to start with ail mammals were capable 

 of hearing very high sounds, and that only later was this capacity reduced in 

 certain groups, notably in the case of the primates. 



The macroscopically observable quantitative différences in the individual 

 sections of the acoustic Systems of various species of whale, are probably related 

 to the bioacoustically determined différences in frequency in the sonagrams. 

 Further research will seek to establish this connection. 



The function of the especially highly developed nucleus ellipticus and the 

 lower olive is still not known. The cerebellum présents further problems. More 

 than half the pontocerebellar fibres end in the powerful paraflocculus. Given 

 the fact that in the pedunculus cerebri of the whale the temporo-occipital projec- 

 tions predominate, the ratio in size between temporal lobe and paraflocculus is 

 very marked. But reliable évidence is still not available. 



It might be supposed that the spécial development of the cerebellum in 

 whales also has a connection with their characteristic powers of hearing. The 

 idea of an " auditory area " in the cerebellum is not new. Snider and Stowell 

 (1944) in experiments with land mammals recorded responses to acoustic stimu- 

 lation in the area of the lobulus simplex and the tuber vermis, in a région which is 

 overlapped by a visual area. When the colliculus inferior is destroyed, thèse areas 

 do not respond to physiological stimuli. 



Snider and Eldred (1948) report that stimulation of the temporal lobe 

 provokes a response in the " auditory area " of the cerebellum and vice versa. 



It still remains to find an answer to the question raised in the Introduction, 

 as to whether the shape of the brain as such is an indication of a pluriphyletic 

 origin of the Cetacea. For the sake of clarity we listed in the last Table (13) the 

 main anatomical différences between Archaeoceti, Odontoceti and Mysticeti. It 

 is in fact a provisional survey, since features which today are considered group 

 spécifie may later turn out to be uncharacteristic in the light of research on other 

 species. Much could be expected, for example, from an extensive study of the 

 comparative ontogenesis of the brain of toothed and whalebone whales. When 

 studying eye development we have noticed how structures which are quite distinct 

 in the early embryonal stages, may, in the course of development and in accordance 



