1030 



G. PILLERI UND M. GIHR 



DISCUSSION AND SUMMARY 



We have seen in broad outline the way in which many brain structures have 

 been modifiée! in a manner which is characteristic for whales by means of spécial 

 physiological adaptations. The most striking thing of ail is the marked develop- 

 ment of the telencephalon. As in the case of land mammals, the cortex is divided 

 into six layers. Unlike many other modifications, this pronounced telencephalisa- 

 tion cannot be considered a resuit of adaptation to life in the water. 



In contrast to the neocortex, the size of the paleocortex, archicortex with 

 Ammons horn and fascia dentata, is very small in comparison with land mammals, 

 including the primates. This gênerai réduction in the size of structures, which 

 today is no longer attributed to the rhinencephalon but rather to the limbic 

 System, which also includes the nucleus anterior thalami, is very striking and can 

 be looked upon as a spécial evolutionary process which only occurred in the case 

 of the Cetacea. The marked réduction in the limbic structures is possibly respon- 

 sible for the decidedly hedonistic nature of the dolphins (Pilleri 1967). The huge 

 expansion of the neocortex in many species of Cetacea explains the highly develop- 

 ed learning capacity and intelligence of thèse animais (Pilleri 1962, Hediger 1963). 



In the case of many species of whales it has been shown that they make use 

 of the echo-sounding (sonar) principle for purposes of orientation by means of 

 ultra-sonic waves. Of the functions of this group of animais, vocalisation and 

 sonar are extremely important, with corresponding development of the whole 

 peripheral and central acoustic System (Fraser and Purves 1954, Kellogg and 

 Kohler 1952, Zvorykin 1963). 



The brain shows markedly high development of the nervus acusticus, the 

 colliculi posteriores, the corpus trapezoideum, the corpus geniculatum mediale 

 and, as central représentative of the auditory tract, of the temporal lobe. 



Kellogg and Kohler (1952) see a connection between the capacity to perceive 

 ultrasonic waves and the huge development of the temporal lobe. But according 

 to the observations of Zvorykin (1963) on the dolphin brain ( Delphinus delphis), 

 it is still not certain whether this capacity is really the resuit of the highly developed 

 temporal lobe. 



On the other hand, the bat, which like the whale orientâtes itself by means of 

 echo sounding, has a lissencephalic brain and its temporal lobe is one of the 

 lowest difTerentiated among the mammals (Rose 1912). According to Zvorykin 

 (1963), maximum ultrasonic capacity is therefore not correlated with the most 

 highly differentiated, but rather with the simplest cortical structure. Herc it 

 might be objected that it seems insufficient to détermine the degrec of differentiation 

 of a cortical area by means of a single staining method like that of Niss/, which 

 docs not show the synapse System and other neuron structures. In Zvorykin's 



