406 J. B. JOHNSTON 



from my own studies. The last point is of great importance. 

 It has been shown in a previous section that the corpus callosum 

 and hippocampal commissure in the opossum, bat, mole, rat and 

 rabbit are everywhere surrounded by the hippocampal forma- 

 tion and the primordium hippocampi. The cells of the indusium 

 verum of ElUot Smith and the cells among which the commis- 

 sure fibers are imbedded must be regarded as the remnant of 

 the cells which enter the lamina supraneuroporica from the adja- 

 cent hippocampal primordia in early stages of the embryo. Thus 

 the anterior pallial commissures in mammals are imbedded in 

 the primordium hippocampi just as they are in selachians. 



STRIAE LANCISII AND INDUSIUM 



To the very concise and illuminating survey of the history of 

 these structures given by Elliot Smith ('97 e) I can add a few 

 interesting comments on the basis of the forms studied. Great 

 differences appear in these structures in various mammals. In 

 most of the rodents the indusium is small and the morphological 

 arrangements seen in the better developed hippocampus are lost. 

 In the woodchuck (Arctomys) there is a fairly thick band of 

 cells continuous across the median line and covered by a layer 

 of fibers. This probably indicates a fusion of the indusium and 

 striae of the two sides. The striped gopher (Spermophilus tri- 

 decemlineatus) agrees with the woodchuck. A very different 

 condition appears in the opossum and the bear. I have seen 

 no mention made of a stria Lancisii in the marsupials and none 

 was to be expected on the hypothesis that this bundle represents 

 fibers which run within the hippocampal formation when that 

 is well developed. Elliot Smith states that in Eutheria "the 

 vestigeal fascia dentata lies in the region of the stria medialis." 

 I was surprised, therefore, to find in transverse sections of the 

 opossum's brain well defined striae mediales on the dorsal surface 

 of the pallial commissure. As shown in figures 29, and 30, these 

 are good-sized dense bundles lying at either side of the middle 

 line and separated from the fascia dentata by the fimbrio-dentate 

 sulcus already described. Traced caudad these bundles curve 

 down on the caudal surface of the commissure and turn laterad. 



