I02 UNIVERSITY OF COLORADO STUDIES 



bent, the relations of Amphibia could only be directly changed into those 

 of higher forms by the migration of the bundle in question across 

 the portas" [interventricular foramens]. (Kingsbury '95, 158.) This 

 peculiar relation, he shows, has either been ignored by other writers or 

 considered of no consequence. He, however, considers the objection a 

 vital one and that the structure under question cannot be homologized 

 with the mammalian callosum, though it may prove to be, in part at 

 least, a hippocampal commissure. 



It appears to the writer that we cannot look to a phylogenetic study 

 of adult structures to settle the point in question. The importance of 

 an embr^'ological study seems to be overlooked. It seems highly prob- 

 able that light would be shed upon this question by a comparative 

 study of the embrj'olog)' of the structures under discussion. Observa- 

 tions have been made upon larval forms, but no careful study upon 

 early embr}^os. Work by cytological methods, comparable to that done 

 upon mammalian and chick embryos might prove of great value. As 

 no such work seems to have been done, a brief discussion and a few 

 suggestions, based chiefly upon conditions observed in mammals and 

 birds may be in place here. 



The account of the origin of the mammalian corpus callosum as 

 given by Minot ('92, 682) is of interest. Early in embryonic life, 

 "the upper part of the lamina terminalis becomes much thickened to 

 form a broad band of triangular section uniting the two hemispheres. 

 This band is the anlage of the septum lucidum, the corpus callosum, 

 the fornix, and the anterior commissure. " At this time there are no fibers 

 connecting the two hemispheres, and in fact, the hemispheres them- 

 selves are but slightly developed. The anlage of each of these commis- 

 sures can be distinguished as a distinct area in this structure. Later, the 

 fibers of these commissures grow across the median line from each side 

 toward the opposite side through their respective areas. Thus we see 

 that the corpus callosum, in its origin, belongs to the lamina terminahs 

 and is not a dorsal structure as held by some writers. The callosum in 

 birds and mammals arises in the lamina terminalis in a position homolo- 

 gous to that occupied by the "callosum" in adult Amphibia. Mrs. 

 Gage's figures 66 and 67, from median sections of embryo Diemyctylus 



