531 



Both are characterised by the presence of curious islands or clumps 

 of closely-packed small cells (Fig. 1). 



In all other Vertebrata, in which the tuberculum olfactorium has 

 been studied, it has been found to be a peculiar cortical formation 

 covering, like a closely-fitting cap, the ventral surface of the corpus 

 striatum. Whether or not it does so in Lepidosiren I cannot say at 

 present, because there is no certain criterion to enable a distinction 

 to be drawn between corpus striatum and tuberculum olfactorium. 

 Appearances seem to suggest — and it is nothing more than a mere 

 suggestion — that during the process of pushing out the great tuber- 

 cular sac the corpus striatum may have been relegated chiefly to the 

 lateral wall and the tuberculum olfactorium to the ventral and mesial 

 walls of the vesicle. But this is mere conjecture. 



What is the meaning of these curious features of the brain? This 

 is a very difficult, perhaps at present, impossible, question to answer 

 satisfactorily. Graham Kerr has remarked that "smell seems to be 

 the best developed of the three ordinary special senses in Lepidosiren"^): 

 hence it is not surprising to find the "smell-brain" highly developed. 



The olfactory apparatus seems to possess varied kinds of usefulness 

 in different classes of vertebrates. Thus the organ of smell becomes 

 useless in the mammal, when in the water: yet most fish possess highly 

 developed olfactory organs, which must be capable of appreciating some 

 kinds of impressions to which the mammalian organ is insensible. 

 Perhaps the Dipnoan, which can breathe and live out of water, may 

 be endowed with organs of smell capable of appreciating stimuli such 

 as those of land animals are fitted to receive, and that this may ex- 

 plain the high state of development of its cerebral hemisphere. But 

 why it should be so more highly organised than the Amphibian cere- 

 bral hemisphere is not altogether clear. 



In the various classes of vertebrates different parts of the olfac- 

 tory apparatus become developed in a predominant degree. In the 

 Dipnoi it is the tuberculum olfactorium, possibly because its intimate 

 association with the corpus striatum — the origin of the basal fore- 

 brain bundle — enables the olfactory organ the more readily to do- 

 minate the rest of the nervous system. In the Reptilia it is the hippo- 

 campal formation, which is specially well- developed, possibly because 

 both smell and sight are well-developed. The visual apparatus — optic 

 lobes — plays the dominant role in Reptilia in controlling the move- 



1) The External Features in the Development of Lepidosiren para- 

 doxa FiTZ. Philos. Transact., Series B, Vol. 192, 1900, p. 303. 



34* 



