652 C. JUDSON HERRICK AND JEANNETTE B. OBENCHAIN 



thalamicus 1 is crossed and the subhippocampal lobe is entered, 

 the cellular structure changes to a more diffusely scattered for- 

 mation throughout the whole of this lobe. Dorsalward this dif- 

 fuse formation is replaced by the hippocampal formation at the 

 site of the sulcus subhippocampalis, and ventralward it is re- 

 placed by the corpus striatum at the sulcus medius (fig. 6). 

 Lateralward the subhippocampal formation becomes continuous 

 with the secondary olfactory area of the posterior lobe of the 

 evaginated cerebral hemisphere. 



From the preceding description it appears that the primordium 

 hippocampi in Ichthyomyzon at no point comes into contact 

 with the sulcus medius nor the underlying primordium of the 

 striatum, but is separated from ' these structures for its entire 

 length by the subhippocampal lobe. This relation is much more 

 evident here than in the 120 mm. specimen previously studied. 

 In this species the primordium hippocampi borders the inter- 

 ventricular foramen to a very slight extent only. The posterior 

 wall of the foramen is formed by the subhippocampal lobe, and 

 the dorsal wall chiefly by the dorsal commissure. The cellular 

 hippocampal formation extends forward into the dorsal commis- 

 sure ridge only very slightly. The primordium hippocampi here 

 does not bend "through the roof of the foramen to become di- 

 rectly continuous with the roof of the hemisphere," as de- 

 scribed by Johnston ('12, p. 354). Unlike the primordial 

 corpus striatum, which does bend through the floor of the 

 foramen into the ventral wall of the evaginated hemisphere, the 

 hippocampal formation is strictly limited to the unevaginated 

 portion of the neural tube. The subhippocampal lobe, on the 

 other hand, is directly continuous with the lobus olfactorius of 

 the evaginated hemisphere. 



The cerebral commissures. The internal structure of this brain 

 has not been exhaustively studied, and only a brief reference will 

 be made to the commissures and decussations shown by the 

 model. The ventral cerebral commissural systems extend for- 

 ward from the floor of the medulla oblongata to the lamina ter- 

 minalis in an unbroken series except at three places, namely, the 

 infundibulum, the postoptic recess and the preoptic recess. 



