cxliv Journal of Comparative Neurology. 



a marked general growth at about the time of final transformation so 

 that the brain much more nearly fills the skull than in the late red 

 forms. 



The morphological suggestions about which questions would nat- 

 urally be raised are those associated with the callosal commissure and 

 the hippocampal structures. While apparently recognizing a hippo- 

 campal lobe by implication the whole mass of the commissure is called 

 callosum and no fornix or hippocampal commissure is recognized. 

 We have already suggested a question as to this homology and think 

 it most probable that the highly osmatic brain would have a structure 

 comparable to the hippocampal or fornix commissure of reptiles. A 

 brief preliminary discussion of the structure has appeared in the Sep- 

 tember number of this journal, p. 124-128. From the illustrations 

 accompanying this article reasons may be gathered for comparing at 

 least a greater part of the so-called callosum with the fornix commis- 

 sure. If analogies with other groups can be employed, it is hard to 

 see how one can neglect the obvious necessity for limiting the region 

 of origin of the callosum to parts lying far cephalad in an osmatic 

 bram. 



The second part of the paper institutes comparison with Amia 

 and the lamprey. Among the interesting observations may be men- 

 tioned the existence in Diemyctylus of a metapore — an opening 

 through the metaplexus into the subarachnoid space. Morphologic- 

 ally considered, the improbability is so great that repeated investiga- 

 tion is well worth making. The section on the cerebral commissures 

 is interesting, especially in the discovery of two commissural bands for 

 the fore brain in Amia. We, however, would regard the ventral one 

 {cm Fig. 103) if actually a commissure, as a homologue of the pre-com- 

 missure and the so-called pre-commissure the representative of the 

 dorsal system (callosum, etc.) Fig. 104 strongly re-enforces this con- 

 jecture, for in that figure /r;// cannot be tortured into an homology with 

 the precommissure of other vertebrates. 



Lying in the " crista," Mrs. Gage finds suggestions of fibres 

 such as the writer has figured on Plate XV, Fig. 5 call, in Menopoma 

 and which might from their position prove a part of the fibres of the 

 callosum or the homologue of the fibres, so-called in fishes. The 

 summary given at the close is repeated here. 



I. A true metapore exists in adult Diemyctylus and indications 

 of it appear in the larvae. In the lamprey and Amia at a correspond- 

 ing part of the metaplexus a sac communicating with the metacoel pro- 

 trudes over the myel. 



