48 DR. G. ELLIOT SMITH ON THE 



cortical areas of these bats with the Marsiijiial type, in order to find out the distinctive 

 mode of arrangement of the former. Having determined the nature of the dorsal 

 commissure of the bat and its relation to the pre-existing type, I shall attempt, 

 by means of comparison with more highly-developed Eutherian forms, to indicate the 

 true lines along which the evolution of the corpus callosum proceeds in the Mammalia. 



For a considerable time I have been aware of the fact that Zuckerkandl, in his 

 remarkable work upon the olfactory centre (' Ueber das Eiechcentritm,' 1887), had 

 grOviped together the brains of Monotremes, Marsupials, and certain small Cheiroptera 

 on the ground that they presented in common certain features in whicli they differed 

 from all other mammals. 



These distinctive features were stated in the following terms : — (1) that theRandbogen 

 [or Randwindung — Gyrus marginalis *] retains its embryonic form, i. e. that it is in all 

 parts equally broad, and extends up to the Ammonsfurche *, and is continuous with the 

 mesial wall of the hemisphere anteriorly ; (2) that the corpus callosum is rudimentary ; 

 and (3) that the Balkenwindung* is wanting. 



From the fact that Zuckerkandl uses the term Randbogen instead of hippocampus we 

 might infer that he did not definitely recognize the dorsal part of the Randbogen as 

 hipjiocampus — an inference which is borne out by his reference to the corpus callosuni. 

 If any other evidence were needed to show that Zuckerkandl quite failed to appreciate 

 the most salient features of the Monotreme and Marsupial cerebrum, it is provided in 

 his remarks upon the cerebrum of Echidna, where he speaks of the fascia dentata as 

 "der ventrale Schenkel des Randbogens." This clearly indicates that he did not 

 recognize the dorsal limb of the Randbogen as fascia dentata or hippocampus. [It is 

 only right to mention that Zuckerkandl's work appears to have been purely macroscopic] 



Thus Zuckerkandl reduces the cerebrum of the Marsupial and Monotreme to the 

 conventional Eutherian type, possessing a true corpus callosum, and with a hippocampus 

 practically limited to the descending limb of the lateral ventricle. Such l)eing the case, 

 until quite recently I paid little heed to his remarks concerning the resemblance of 

 Vespertilio murinus (the bat of which he examined the brain) to the Marsupial type ; for, 

 having reduced the Marsupial to conformity with a simple Eutherian type, it seemed very 

 natural to group an actual example of such a type (wliich the bat provides) with the; 

 Marsupial. 



But quite recently, vipon examining the brain of a common Australian bat {Nycto- 

 •philus Timoriensis), I was struck by the remarkable resemblance which its hippocampus 

 (Zuckerkandl's Randbogen) and cerebral commissures present to the Metatherian 

 type. It appeared to be a case of reducing the bat to the Metatherian condition 

 rather than advancing it to the Eutherian status, as Zuckerkandl's researches would 

 appear to indicate. In the course of this communication we shall see how far this 

 appearance is justified by the histological examination of NyctopMlus and liinioplenis. 



The object of this paper being to consider phylogenetically the origin of the true 

 corpus callosum (as distinct from the hippocampal commissure), we are solely concerned 

 with the transition from the condition found in the Proto- and Metatheria, in whicli 



* These terms will le explained subsequently. 



