The origin of the Corpus Callosuro, a contrib. upon the Cerebral Comm. etc. 245 



of the pars olfactoria, although we have still to account for the 

 general lack of commissural supply of the mantle. 



Mammalia. 



I have little to add to the observations of Sander and Ganser 

 upon the adult relations of the anterior commissure in the higher 

 mammalia. In horizontal sections of the brain of a foetal guinea- 

 pig {Cavia) the divisions of this commissure are well displayed (fig. 6) , 

 The pars olfacteiia forms the upper and the pars temporalis the 

 lower tract. They are united in the centre and divide laterally in 

 front of the cerebral peduncles. The relations are practically similar 

 to those in the Amphibian and Chelonian brain. 



At Professor Kupffer's suggestion I began to follow the embryonic 

 development of the cerebral commissures in the sheep's brain, reviewing 

 tho work of Mihalkovics. The following are some of the first results. 

 The anterior commissure and corpus callosum fibres both appear lat- 

 terally, that is in the corpus striatum and corona radiata region respec- 

 tively, before they appear in the median plane. The fibres then extend 

 towards the centre. Before either bundle unites with its fellow in the 

 opposite hemisphere, the descending fibres of the fornix appear between 

 them. The opposed inner surfaces of the hemispheres commence to 

 unite at the primary lamina terminalis and this union extends forwards. 

 While this union is still imperfect the fibres of the anterior commis- 

 sure approach the centre and unite. Somewhat later, the callosal 

 fibres seem to unite in the centre in the same manner. It thus appears 

 that the corpus callosum and anterior commissure follow a similar line 

 of development, and, to our present knowledge there is nothing in 

 their development which is contradictory to the theory that both of 

 these commissures are primitive characters of the vertebrate brain. 



I hope at some future time that, either by some other investi- 

 gator or by myself, the preceding study may be completed and the 

 whole history of the cerebral commissures followed from their origin. 

 The gap I have left is between the fishes and amphibia. I have 

 endeavored in each instance to indicate the degree of certainty at- 

 tached to the foregoing observations, upon which the interpretation 

 of the various homologies rests. Of the course of the upper 

 bundle in the Amphibia and Chelonia there is no doubt. The pars 

 olfactoria in the Sauropsida remains to be more thoroughly studied. 



