656 C. JUDSON HERRICK AND JE ANNETTE B. OBENCHAIN 



this applies to all of the true fishes also) is as yet too meager to 

 warrant any but provisional morphological interpretations. 



Recent investigations, particularly those of Sterzi and John- 

 ston, have established with a high degree of probability the ho- 

 mologies of the subdivision^ of the telencephalon of cyclostomes. 

 The evaginated cerebral hemisphere includes an anterior lobe, 

 the olfactory bulb, and a posterior lobe containing a portion of 

 the secondary olfactory area and a portion of the primordium of 

 the corpus striatum. The secondary olfactory area (nucleus 

 olfactorius) of mammals has three divisions, lateral, medial and 

 intermediate. The intermediate nucleus, or tuberculum olfac- 

 torium (lobus paroKactorius of Edinger), has not been identified 

 in cyclostomes. The medial nucleus is represented in the ven- 

 tro-medial part of the cyclostome hemisphere and in a portion 

 of the unevaginated telencephalon medium between this region 

 and the preoptic recess. The lateral olfactory nucleus is repre- 

 sented in the posterior lobe of the hemisphere, though it cannot 

 be maintained that these two stuctures are strictly homologous. 



The primordial striatum lies in the floor of the interventricular 

 foramen partly within the telencephalon medium and partly with- 

 in the hemisphere, exactly as in young amphibian larvae. The 

 remainder of the telencephalon lies entirely in the unevaginated 

 portion of the neural tube and includes the preoptic nucleus, the 

 primordium hippocampi and the lobus subhippocampalis. The 

 first of these structures remains throughout the vertebrate series 

 in the telencephalon medium; the other two are variously arranged 

 in different species of fishes, and in amniote vertebrates are firlly 

 evaginated in the cerebral hemisphere. 



The structure here termed 'lobus subhippocampalis' was first 

 described by Herrick ('10, p. 472) as the rostral end of the pars 

 dorsalis thalami, the overlying primordium hippocampi there be- 

 ing termed the dorso-median ridge. In the 120 mm. specimen of 

 Ichthyomyzon there described no sulcus was found separating 

 these two regions, but they were distinguished on the basis of 

 internal structure. Johnston ('12) in reexamining the same ma- 

 terial did not observe this internal differentiation and described 

 both regions as primordium hippocampi, whose ventral boundary 



