i8o Journal of Comparative Neurology. 



paper by the writer in 1890/ a rudiment of the fornix was lo- 

 cated in the alHgator. More recent examination fails to deter- 

 mine any definite commissure in the region. It will be noticed 

 that the callosum is larger than the pre-commissure and lies in 

 the very cephalic part of the terma. In the case of the alligator 

 it should be remembered that in dealing with the brains of speci- 

 mens 1 2 to 18 inches long, we have practically embryonic material 

 before us. The olfactory tubers will ultimately separate from the 

 brain and it is not improbable that new conditions may arise. 



VIII. TJie Jiippocampal region of Opossum. Figs. 9-12, 

 of Plate XXVII, illustrate the appearance of several cross sec- 

 tions of an embryo from the same litter of pouch embryos. 

 The relations of the hippocampus are absolutely simple.^ 



One recent author^ calls in question not only the homolo- 

 gies of the gyrus dentatus but is "absolutely opposed, however, 

 ... to all anatomists with whose works I am acquainted, in 

 their attempts to homologize the elements found in the fascia 

 dentata with those found in other parts of the cortex ; or, in 

 other words, to prove that the fascia dentata is a convolution 

 comparable with other convolutions of the general cortex. 

 Firstly, because in structure it does not appear to me to resem- 

 ble the general cortex, and, secondly, because it does not behave 

 like a convolution. For example, it does not vary in its devel- 

 opment with the convolutions of the general surface." It 

 seems hard to understand how such distinctions as Dr. Hill 

 makes can be maintained in the face of accumulating embryo- 

 logical evidence. In tracing the development of the caudo- 

 mesal border of the mantle one may follow step by step the 

 gradual differentiation of the mantle border. The cellular bor- 

 der is for a time homogeneous, but gradually is thrown into folds 



1 Notes on the Brain of the Alligator, ybwrw. Cincin. Soc. Nat. Hist. 



2 Obscurity was needlessly introduced in the figures of Plate A, Vol. II, of 

 this journal, by using the terras gyrus fornicatus and uncinatus for parts which 

 are more often named cornu ammonis and gyrus dentatus respectively, without a 

 discussion of homologies concerned. 



3 Hill, A. The Hippocampus. Philos. Trans., CLXXXIV, B. pp. 30-429. 



