34 BULLETIN OF THE LABORATORIES 



are narrowly flask-shaped or fusiform. Thedifference is emphasized 

 in the sections by the fact that the gyrus uncinatus lies in a different 

 plane from the gyrus fornicatus, and thus when the cells of the latter 

 are cut longitudinally those of the former are almost always cut trans- 

 versely. Perpendicular longitudinal sections of the mouse brain, how- 

 ever, show the two laminae in much closer contact, and in the opos- 

 sum the continuity is unbroken, especially cephalad, though in the 

 caudal portion the arrangement is more like that of rodents. Old em- 

 bryos of the rabbit, too, show the formation of the folds above 

 described with great clearness. The gyrus uncinatus in these rodents 

 thus appears to be more or less displaced, as well as folded. In the 

 rabbit embryos referred to it would appear to originate as a thickening 

 on the free edge of the gyrus fornicatus, which is thrust back as by 

 some external resistance, both on the ectal and ental surfaces of this 

 edge, but chiefly on the latter. The cells which connect the two por- 

 tions at this stage appear to be subsequently lost or to be replaced by 

 fibrous elements. 



The confluence of the gray matter of the two hippocampi, which 

 Stieda describes in the case of the mouse, does not occur here. The 

 external fibre zones of the two sides do come into contact in the 

 meson, but this is obviously merely a mechanical adhesion. I suspect 

 that the confluence which Stieda observed was more apparent than 

 real, though my own mouse sections show the two sides in much 

 closer contact than in Geomys. Fiber zibethicus resembles the mouse 

 in this respect, and in the configuration of the hippocampus as a 

 whole. 



Erethizon dorsatus presents a structure essentially like that de- 

 scribed for Geomys, though somewhat more difficult to follow. The 

 chief difference lies in the gyrus uncinatus, which is larger and over- 

 laps the free margin of the gyrus fornicatus much more than in 

 Geomys. In Arctomys monax there is considerable variation, though 

 only in the relative position of the parts. It is illustrated quite fully 

 in Vol. V, Plate IV, Fig. i, .Plate V, Figs, i, 2, and 3, Plate VII, 

 Fig. 2, Plate X, Figs. 5 and 7, Plate XIX, Figs. 1 and 2. 



T>iencephalon and Mesencephalon. The relations here are essen- 

 tially as in the rat. The thalamus is much smaller than in Erethizon, 

 the cephalo-dorsal portion especially being abbreviated. Thecinereum 

 is nearly circular with the infundibulum passing through the centre. 

 The chiasm is not so prominent as in Erethizon, though essentially 



