390 J. B. JOHNSTON 



the islands of Calleja and it is readily seen that they belong 

 with the tuberculum olfactorium in which such islands are nu- 

 merous. I have never seen these islands rising in the medial 

 wall quite as high as the nucleus parolfactorius lateralis. Medial 

 to these is a more diffuse layer of cells which is continuous ven- 

 trally with the superficial layer of the tuberculum external to 

 the islands. This is the nucleus parolfactorius medialis. It is 

 evident that both this and the islands constitute a continuation 

 of the tuberculum olfactorium into the medial wall, as the writer 

 has pointed out in the case of selachians ('11 a). The boundary 

 line between the primordium hippocampi and these two nuclei 

 in the medial wall is somewhat V-shaped as indicated in the 

 figures. The medial nucleus varies greatly in extent but its cells 

 are always imbedded among the fibers of the fasciculus prae- 

 commissuralis, forming a more or less dense superficial plate or 

 layer. In the mole and also in the rabbit it comes up from in 

 front almost to the fornix columns; in the bat it rises somewhat 

 higher than the nucleus lateralis, but a large primordium hippo- 

 campi intervenes between it and the fornix and the corpus cal- 

 losum. In the rat the nucleus medialis is small and does not 

 extend as far rostrad as in most forms. In the turtle the nucleus 

 medialis covers the outer surface of the nucleus lateralis with 

 diffusely scattered cells as in the selachian and frog. 



The section through the genu in the bear's brain presents a 

 very different appearance from those described above, owing to 

 the great development of the frontal lobe which occupies the 

 space in this section between the olfactory bulb and the genu. 

 In figure 73, however, is drawn a section between the genu and 

 lamina terminalis in which the relations of the primordium hip- 

 pocampi, nucleus parolfactorius lateralis and nucleus parolfac- 

 torius medialis are seen to be as described above. In this sec- 

 tion the indusium appears above the corpus callosum and the 

 corresponding cord of gray beneath it. 



Section through the neuroporic recess. Figures 17, 29, 44, 51, 

 62, 69, 70, 72. This section cuts at the same time the anterior 

 commissure, the fornix columns and some part of the anterior 

 pallial commissure complex, and passes just rostral to thefora- 



