The origin of the Corpus Callosum, a contrib. upon the Cerebral Comin. etc. 243 



vity. This renders it very difficult to follow. I prepared five series 

 of sections in slightly different horizontal planes before reaching any 

 positive result. The bundle is divided into two courses of fibres, 

 an upper and lower. In horizontal sections fig. 17), passing from 

 above downwards we observe first that all the fibres are directed 

 slightly forwards; these constitute the upper course. We observe 

 next that in lower sections that all the fibres are directed backwards 

 and may be followed around the posterior horn of the lateral ven- 

 tricle; these constitute the lower course. In transverse sections (fig. 16), 

 a similar division is observed, part of the bundle passing upwards 

 beneath the ventricle, part passing directly outwards. The division 

 into two tracts is much less plainly marked than in either the Am- 

 phibia or Mammalia. The postero-inferior course undoubtedly repre- 

 sents the pars temporalis of the anterior commissure. The anterio- 

 superior probably represents the pars olfactoria, which is here 

 the smaller division. 



The chelonian brain thus supports the homologies I have sug- 

 gested for the commissures in the Amphibia. The distribution of the 

 upper bundle to the inner mantle is much more readily observed. 

 The division of the lower bundle into two tracts is also demonstrated 

 and the fact that the lower tract clearly supplies the temporal lobes 

 while the upper tract turns upwards and forwards strengthens the 

 view that the lesser division of the lower bundle in the Amphibia 

 is a rudimentary pars temporalis. There are three differences with 

 the Amphibian brain to be noted, first, the change in position of the 

 commissures; second, the great development of the pars temporalis; 

 third, the reduction of the Pars olfactoria. The two former are in 

 the direction of the mammalian tj'pe; the latter seems to be a cha- 

 racteristic of the Sauropsida which is still more marked in the birds. 



My observations upon the upper bundle accord with Stieda's 

 and Kabl-Kückhard's earlier views (187S), although I cannot accept 

 the homologies suggested by the latter author. I have not examined 

 the brain of Psammosaurus and so cannot express an opinion as 

 to the presence of the chiasma partes olfactoriae although this 

 observation is opposed to the facts of the distribution of the upi)er 

 bundle in the Chelonia. As to the bundle which Rabl-Rücivhard sug- 

 gests is a rudiment of the fornix, there are several objections to this 

 view. In the Amphibia this bundle is uniformly present ^fig. 7 acfn) : 

 part of its fibres I have observed (1S84 fig. 8) descend into the outer 

 part of the optic thalami, part pass into the hemispheres and extend 



Morpholog. Jahrbuch. 12. 17 



