412 J. B. JOHNSTON 



In the medial wall of the frog brain the sulcus and cell-free zona 

 which have commanded the attention of all workers seems at 

 once to divide the pallial from the basal areas and to correspond 

 to the sulcus and zona limitans hippocampi in the selachian 

 brain. It must be noticed, however, that a zona limitans, whether 

 accompanied by a sulcus or not is usually the expression of a 

 rearrangement of neurones in the adjacent areas due to func- 

 tional or mechanical causes or both. In the selachian the zona 

 limitans in the medial wall marks the line of meeting of simple 

 olfactory with the olfac to-gustatory correlating centers. The 

 line of demarcation between the two was perhaps determined 

 by the connection of the neural tube with the ectoderm at the 

 neuroporic recess. In the amphibian a second differentiation 

 has begun and is well advanced in the frog, namely, the formation 

 of hippocampus. This is marked by a change of form and rear- 

 rangement of cells in the pallium. The cells are now pyramidal 

 and have their long axes placed vertically to the brain surface 

 instead of being wholly without arrangement. Such a change 

 constitutes a very marked difference from the selachian brain 

 and leads to the formation of a new zona limitans. Below this 

 zona limitans the neurones are scattered without any regularity. 

 These facts, however, are not sufficient to show that the zona 

 limitans separates pallial and basal areas. Immediately below 

 the zona limitans appears a group of large cells which stand out 

 conspicuously from the cells of the remainder of thisregion ('11 a, 

 fig. 75, Herrick '10, fig. 40, n. medialis septi). When these cells 

 are examined it is found that they form a column extending 

 forward to the olfactory peduncle. This column of cells corre- 

 sponds to the ''septum" or primordium hippocampi of the turtle's 

 brain. In the rather broad and shallow sulcus between it and 

 the hippocampus runs the fimbria, as is well known. 



Traced caudally this column of cells continues above the fora- 

 men interventriculare where it is separated from the developing 

 hippocampus by a ventricular sulcus and a cell-free zone. This 

 portion is known as the pars fimbrialis (Kappers) or the supra- 

 foraminal portion of the paraterminal body (Herrick). The fim- 

 bria runs in this body to be distributed to the posterior part 



