388 J. B. JOHNSTON 



as in the opossum. The same is true in the sheep and foetal 

 dog. The area parolfactoria is smaller in all of these than in 

 the turtle or the opossum, and in some other mammals examined 

 (cat, adult dog, man) this ridge fails to appear in the ventricle. 

 This is due in part to its smaller size (man especially) and in 

 part to the secondary obliteration of the ventricle between the 

 area parolfactoria and the head of the caudate nucleus. In these 

 forms the primordium hippocampi seems to extend down in the 

 medial wall to the floor of the ventricle. 



INTERNAL STRUCTURE OF THE MEDIAL WALL 



To illustrate the relations in the internal structure of the me- 

 dial wall of the hemisphere several transverse sections are drawn 

 from the brains of the turtle, opossum, bat, rabbit, rat, mole 

 and bear. 



Section through the olfactory peduncle. Figures 12, 25, 41, 48, 

 56, 65, 75. The matter of interest in this section is the fact 

 already well known from the work of Elliot Smith and others, 

 that the hippocampal formation extends rostrad almost to the 

 formatio olfactoria in most reptiles and lower mammals. The 

 sections selected do not pass through the extreme rostral end 

 of the hippocampal formation but are taken either through the 

 olfactory formation or very close caudal to it. In each section 

 there is seen beneath the ventricle the head of the caudate nu- 

 cleus, covered ventrally by a thicker or thinner tuberculum olfac- 

 torium. The cells of the tuberculum extend up a short distance 

 into the medial wall. Dorsal to this appears a mass of cells 

 which are without regular arrangement and usually are smaller 

 and paler than those in adjacent nuclei. This is the body which 

 has been identified in the above pages as the primordium hippo- 

 campi. Above this is a dense mass of deeply staining cells which 

 is continuous caudad with the supra-callosal hippocampus in mam- 

 mals and with the medio-dorsal cortex in reptiles. This is the 

 rostral end of the hippocampal formation. 



Section near genu corporis callosi. Figures 42, 50, 58, 66, 75, 

 77. Since it is impossible to compare any particular section of 

 the turtle's brain with a section through the genu of the corpus 



