650 C. JUDSON HERRICK AND JEANNETTE B. OBENCHAIN 



At the level of figure 6 there is a narrow zone of small deeply 

 staining cells between the preoptic nucleus and the corpus stri- 

 atum which increases rapidly in width as it is followed forward. 

 This is the nucleus olfactorius medialis (figs. 3, 5, nuc.'ol.m.), which 

 in front of the foramen becomes continuous with the great lobus 

 olfactorius which composes the posterior part of the cerebral 

 hemisphere. 



The floor of the interventricular foramen is formed chiefly by 

 the striatum, but in part anteriorly by the nucleus olfactorius 

 medialis. Its rostral boundary is formed by the lamina termi- 

 nalis and by a lobule which is said by Johnston to contain bulbar 

 formation (doubtless correctly; our preparations do not permit 

 a clear analysis of this structure). The roof of the foramen is 

 formed chiefly by the fibers of the dorsal commissure, and its 

 posterior wall by the lobus subhippocampalis, which is described 

 below. 



The hypothalamus. The exact dorsal boundary of the hypo- 

 thalamus of cyclostomes is difficult to define on the basis of our 

 present knowledge. This part of the brain evidently includes 

 ' the greater part of the region below the chiasma ridge and the 

 tuberculum posterius, its dorsal limit lying probably at about 

 the level of the sulcus ventralis. It includes the walls of the 

 postoptic recess, infundibular sac and corpus mammillare, and 

 a massive lateral area which probably corresponds in part with 

 the inferior lobes of higher fishes. 



The primordium hippocampi. The structure and homologies 

 of this part of the cyclostome brain have been very clearly es- 

 tablished by Johnston in his recent paper ('12). He has failed, 

 however, in our opinion, to determine precisely its ventral bound- 

 ary and this has led to some errors of interpretation. 



The primordium hippocampi forms the dorsal part of the mas- 

 sive brain wall between the epithalamus and the dorsal commis- 

 sure in the lamina supraneuroporica. Its dorsal border is marked 

 by a ridge containing nerve fibers of diverse sorts. Some of 

 these connect in front with the dorsal commissure and some be- 

 hind with the habenula. The membranous roof is attached to 

 this ridge and if, as Johnston maintains, the whole of the pri- 



