658 C. JUDSON HERRICK AND JEANNETTE B. OBENCHAIN 



cyclostomes has been emphasized by SchilUng ('07, p. 432) and 

 is evident from our preparations, but our material is inadequate 

 for an analysis of these relations. 



The most important landmark here is undoubtedly the sulcus 

 limitans of His, whose diencephalic portion is not, however, 

 clearly preserved in our specimen. As already stated (p. 642), 

 this sulcus appears to be coextensive with the posterior part of 

 our sulcus medius and farther anteriorly with a shallow depres- 

 sion which follows the upper border of the chiasma ridge into the 

 preoptic recess. In the mid-thalamic region this sulcus is entirely 

 interrupted by an eminence which is here termed the lobus ven- 

 tralis thalami. The relations of this lobe are substantially iden- 

 tical so far as known with those of the pars ventralis thalami of 

 amphibian larvae (Herrick '10) and we incline to regard these 

 structures as homologous. In both cases this eminence is formed 

 by the multiplication and differentiation of subependymal neu- 

 rones in a region which crosses the site of the primary sulcus lim- 

 itans, which is thereby obliterated. Here it clearly lies chiefly 

 below the site of the sulcus, which was doubtless its primitive 

 position; that is, it was first differentiated in the ventro-lateral 

 or motor lamina of the neural tube. But even in this cyclostome 

 it has extended sufficiently far dorsalward to obliterate the sulcus 

 in this region — a process of differentiation which appears to 

 have advanced much farther in the Amphibia. This stucture lies 

 for the most part behind the region designated pars ven. thai, in 

 our earlier account (Herrick '10, fig. 73) the latter referring chiefly 

 to the posterior part of the corpus striatum and to an intervening 

 undifferentiated area. The comparison of any part of this struc- 

 ture with the eminentia thalami of urodeles (cf, Herrick '10, p. 

 471) is obviously untenable, as Johnston has pointed out ('12, 

 p. 349). 



Johnston describes ('12, p. 347) in Ichthyomyzon, as in other 

 cyclostomes, a crescentic sulcus hypothalamicus extending from 

 the interventricular foramen backward and downward into the 

 hypothalamus. There is no such continuous sulcus on either 

 side of our specimen, the horizonal limb and vertical Hmb of 

 Johnston's sulcus being entirely interrrupted by the lobus ven- 



