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C. Tiídson Heirick 



salis. This leads to a heaping up of tissue which projects into the ven- 

 tricle and a rolling of the dorsal border (taenia) laterahvard and finally 

 ventralward (figs. 26, 27). This eversión is a secondary movement which 

 takes place late in development after the ventral part of the lateral wall 

 has been strengthened by the presence of the massive basal forebrain 

 bundles. The more plástic dorsal tissue flows over the more stable base 

 with consequent shifiing of the relations of the mobile parts to the more 



stable basal parts. Evagination of the dipnoan 

 and amphibian type, by a lateral buckling or 

 out-pouching of the entire wall, is here impos- 

 sible. The latter movement can occur only in a 

 uniformly thin wall or in early developmental 

 stages before the ventral part of the wall has 

 been strengthened by massive fiber tracts. 



In teleosts the primordium hippocampi of 

 Holmgren (fig. 27, I) has the one point of re- 

 semblance to the hippocampus of higher brains, 

 that it borders the attachment of the membra- 

 nous roof (taenia). But the taenia has shifted 

 to an atypical position, and in internal struc- 

 ture and conections this lateral part of the área 

 olfactoria dorsalis ifigs. 23, 24, a. ol. d. 1.) much 

 more closely resembles lobus pyriformis than 

 hippocampus. In reality it is not directly ho- 

 mologous witheither of these parts, for its spe- 

 cific internal structure has been developed in 

 response to functional connections determined by its topographic rela- 

 tions, and these are unique. 



A consideration of the relations of the tractus pallii leads to similar 

 conclusions. In all fishes the tractus pallii enters the área olfactoria dor- 

 salis at the lateral border and distributes throughout this región. This 

 connection between the hypothalamus and the dorsal área is the feature 

 upon which Johnston lays most stress in determining the homology of 

 the latter área with the hippocampus of higher forms. But the hypothala- 

 mic connection of the hippocampus is by a medial, not a lateral tract, 

 namely the fornix. The mammalian equivalent of the tractus pallii (which 

 is the olfactory projection tract of Cajal) does not connect with the 



28 



Fig. 28. — Transverse sec- 

 tion through the cerebral 

 hemispheres of adult Am- 

 blystoma tigrinum at the 

 level of the foramen ínter 

 ventriculare, with conven- 

 tional markings for com- 

 parison with figures 26 

 and 27, to illustrate the 

 theory that the primordial 

 hippocampus of Amphibia 

 has been derived from the 

 área olfactoria dorsalis bv 

 inversión of the dorsal Up. 

 This view is regarded by 

 the author as not suppor- 

 ted bv the facts. 



