CEREBRAL COMMISSURES IN THE VERTEBRATA. 



473 



suggestions utterly nugatory. In addition, the fact that Spitzka called the commissure of his " hipjjo- 

 campi " "corpus ca//osiun " * seems to indicate a distrust of his own suggestions. 



In 1888 I'^dinger definitely homologized the medio-dorsal cortical formation in a number of Reptiles 

 as the " Ammo mr hide," or " Ammonst'ormation," and in a series of excellent figures of coronal sections of 

 the brain of a Saurian he leaves no doubt as to the meaning of ids written statements ; for he labels the 

 medio-dorsal cortex in these illustrations " Cortex Ammonis " \. It is instructive to quote his reasons 

 for this conclusion, because they are logical and well-founded, and have a considerable interest in view 

 of the fact that their author was led into a singular error five years later and gave up his earlier and 

 correct belief for another view. In 1888 he wrote {up. cit. p. 116): — "Das betrefFende Rindeastiick 

 [medio-dorsal cortex] muss man als orstes Auftrcten der Ammonsrinde [hippocampus] in der Tierreihe 

 ansehen. Die Ammonsformation wird characterisirt durch ihrc Lage iiber dem Hemisphiirenrand, 

 durch den ilir anlagerndcn Fornix uud durch den auf ihn folgenden Plexus choroides. Sic enthlilt also 

 bei ihrem ersten auftrcten alle die Elemeute welche spater, wenn auch durch Windungen und 

 dergleichen verwischt, das charactcrische Ammonshorn bilden." 



Fis. 20. 



Part of a transverse section through the cerebrum oi Hydvosnurus (immediately in front of the commissures), 



showing the decussating fornix-fibres (a). 



This is not the first representative of the cornu Ammonis in the animal series, for it can be distinctly 

 recognized in a large number of the lehthyopsida ; nor is it accurate to speak of part of the paraterminal 

 body as the " fornix." Rut in spite of these inaccuracies, Edinger^s argument is perfectly legitimate ; 

 for the position [at least of the caudal part] of the medio-dorsal cortex in the mesial wall of the 



* According to Kdiuger, Abhandl. d. Senckeuberg. (icsellschaft, 1^96, p. 314. 



t L. Edinger, " Untersuchungen iiber die vergleichende Aiiatomie des (Jehirns— I. Das Vorderhirn," Abhand- 

 lungen der Senckenbergischen naturforschendea Gesellschaft, 1888, Taf. iii. figs. 21, 22, 23, and 24. 



G9* 



