The origin of the Corpus callosuiu, a contrib. upon the cerebral coium. etc. 535 



Deyelopment of the Cerebral Coramissures. 



I began to study the development of the cerebral commissures 

 with a view of ascertaining how far the embryological history re- 

 peats the ancestral history. In course of this study, not only were 

 my previous conclusions confirmed but several new facts were dis- 

 covered. 



Amphibia. In the frog's brain I find that the upper bundle 

 or corpus callosum is developed at an early period when the fora- 

 mina of Monro are still widely open and in a primitive condition. 

 The fibres of the anterior commissure appear slightly later as a de- 

 licate transverse thread immediately below the corpus callosum. The 

 fact that the upper bundle is developed a little earlier than, or si- 

 multaneously with, the lower is reversed in the mammals, where, 

 according to Mihalkovics' (1877 p. 124 , and my own observations, 

 the anterior commissure is formed earlier than the corpus callosum. 

 A mechanical explanation of this exception is found in what we may 

 call the primary and secondary positions of the corpus callosum. In 

 the primary position, Amphibia (and Fishes ?, this commissure lies 

 in the floor of the primary fore-brain, a part of the brain wall which 

 is early developed, hence its developement is as rapid as that of 

 the anterior commissure. In the secondary position. Mammalia, 

 the corpus callosum lies in the middle portion of the terminal plate 

 which is a later growth than the floor of the ventriculus communis, 

 hence its late development. 



Mammalia. The series of sheep embryos which formed my 

 material, extended from 3.0 to 5.2 centimeters in length. In the 

 latter the corpus callosum was distinctly formed. The following is 

 the order of development. 



In the 30 mm stage the hemispheres have already partially 

 united in front of the primitive lamina terminalis forming the ter- 

 minal plate. The anterior commissure now appears as a delicate 

 thread of fibres in the lateral region of the brain stem. The hippo- 

 campal sulcus is well marked. At 35 mm the anterior commissure 

 extends slightly nearer the median line. In an Embryo of 37 mm 

 the terminal plate has extended considerably forwards. The an- 

 terior commissure shows a division into the pars olfactoria and tem- 

 poralis, while in the median line its fibres begin to unite with those 

 of the opposite hemisphere. This union does not take place in the 

 terminal plate, as stated by Mihalkovics, but in front of it, i. e. 



