404 J. B. JOHNSTON 



mammalian corpus callosum. It is worthy of note that Unger 

 ('06) describes in the gecko a large part of the dorsal commis- 

 sure going to the cortex far lateral to the hippocampus. These 

 fibers can scarcely be other than corpus callosum fibers. Pedro 

 Ramon ('94) has also figured fibers in Lacerta, which must be 

 callosal fibers if the figure is accurate. See Elliot Smith's dis- 

 cussion of this ('03, p. 482). See also Cajal ('04, p. 1103), who 

 states that these are callosal fibers. In the turtles studied the 

 lack of medullation in the dorsal commissure has made it impos- 

 sible thus far to secure positive evidence as to the presence of 

 callosal fibers. 



DEVELOPMENT OF PALLIAL COMMISSURES 



The development of the pallial commissures has been the sub- 

 ject of extensive phylogenetic and ontogenetic studies by numer- 

 our authors. For the general course of evolution of the two com- 

 missures we are indebted chiefly to the comparative researches 

 of Elliot Smith. The studies of embryonic development by 

 Schmidt ('62), Mihalkovics ('77), Blumenau ('91), Marchand 

 ('91), Martin ('93), His ('89, '04), Hochstetter ('98), Zuckerkandl 

 ('01, '09), Gronberg ('01), and Goldstein ('03), have given con- 

 flicting results on certain points. Elliot Smith maintained that 

 the fibers of the co|pus callosum in mammals entered the com- 

 missure bed which already contained the hippocampal commis- 

 sure, that the callosal fibers became segregated in the rostral 

 limb of a crescent-shaped commissure, that the growth of the 

 general cortex was followed by an increase of the callosal fi.bers, 

 that these fibers caused an expansion and stretching of the com- 

 missure bed and that the entire hippocampal-callosal commissure 

 system remains in higher mammals surrounded by the vestiges 

 of the primary commissure bed. Mihalkovics, His and Zucker- 

 kandl have held that the corpus callosum forms in a secondary 

 area of fusion of the medial walls of the hemispheres. This is 

 opposed by Goldstein who believes that in man the primary 

 commissure bed is expanded by the callosal fibers growing into 

 it. There has just come to hand as I write the study of Werk- 

 man ('13) who finds this to be true in Vesperugo, Erinaceus, and 

 Talpa. The writer has studied carefully the development of the 



