Fiinctional factors iii the morphology of the forebrain of fishes 183 



the adult Amia or teleost. llie arrangement of the numbers I, 2 and 3 in 

 the two diagrams indicates that, according to this view, the dorsal border 

 or taenia, I, has been mechanically bent laterally resulting in an eversión 

 of the área olfactoria dorsahs without change in the morphological rela- 

 tionships of its parts. As developed by Holmgren, this theory regards 

 región I as primordial hippocampus, región 2 as primordial general cor- 

 tex, and región 3 as primordial pyriform lobe. Johnston (1911 a, 1912 a) 

 adopts a similar view of the case, but 

 the mammalian homologies of the parts 

 are differently conceived. 



On this view the amphibian cere- 

 bral hemisphere has been derived from 

 the form shown in figure 26 by an in- 

 versión of the dorsal borders of the la- 

 teral walls followed by an evagination 

 of the middle part of the wall. In 

 accordance .with the plan of subdivi- 

 sión of the telencephalon adopted in 

 this paper, this would result in the 

 form of cerebral hemisphere shown in 

 figure 28 and the parts bearing corres- 



ponding numbers in figures 26, 27, and 28 would be homologous. It 

 should be added that the schemata of teleostean and amphibian brains 

 given by both Johnston and Holmgren to illustrate this principie of ever- 

 sión and inversión differ from those here presented because they have 

 different ideas of the mammalian homologies of the parts of the forebrain 

 ■of fishes, but they agree in tlieir views of the morphogenetic principies 

 involved. 



This view of the case has the merit of simplicity and is very attractive 

 but in my opinión it is too simple to fit the facts. In the teleost (fig. 2/) 

 the área, I, which borders the taenia is regarded by Holmgren as primor- 

 dial hippocampus and the dorsal área, 3, as primordial pyritorm lobe. 

 Neither the development ñor the infernal structure of these brains 

 supports this view, but on the contrary they speak decisively against it. 



In the development of the everted form proliferation of neuroblasts 

 occurs (as always in lower brains) on the ventricular side, and in most Te- 

 leostei the greatest proliferation is in the región of the área olfactoria dor- 



--f. med.t. 



27 



Fig. 27. — Similar section through 

 the brain of adult Amia or teleost, 

 to illustrate the theory that this 

 form was derived from that shown 

 in figure 26 by simple lateral bend- 

 ing or eversión of the dorsal part 

 of the wall. The numbers, on this 

 view, would represent correspond- 

 ing áreas. 



