252 FISH. [Vol. X. 



Wilder {57) has described it in the Ceratodtis, a dipnoan, but 

 finds that its direction is toward the cephalic end of the brain, 

 which may be due to the ventral position of the cerebrum. 

 With a possible exception of a few reptiles, the rima does not 

 distinctly appear again until the birds, where it is but slightly 

 developed. In mammals it reaches its highest development, 

 extending from the porta to near the tip of the temporal lobe; 

 and the Amphibia, birds, and mammals seem to form a series, 

 showing the essential stages of the development of this part. 



In the relatively long and narrow amphibian brain, with the 

 cavities about all on the same level, and with the supraplexus 

 dipping down through the roof, there is little need of a rima; 

 but in the case of the birds, where there is more compactness, 

 where many of the parts have widened and overlapped certain 

 of the other parts, radical changes must take place with regard 

 to the plexus and its relations. From the position of the 

 plexus and of the rima and its degree of development in the 

 three forms above mentioned, a most natural inference would 

 be that the rima is caused by the plexus beginning at the porta 

 and cutting its way along the floor of the paracoele toward the 

 side of each hemicerebrum; but the plexus, covered by its 

 lining epithelium, — the endyma, — effectually shuts off any 

 natural communication between the two, and the porta is 

 entirely circumscribed by the endyma. It is not improbable 

 that several conditions are concerned in bringing about the 

 rima, that not only the lateral growth of the parts, but the 

 cranial flexure and great development of the callosum are 

 likewise involved. 



Paraphysis. — The paraphysis is an. outgrowth in the roof of 

 the brain cavity, and it is variously said to belong to the 

 prosencephal and diencephal. It is enclosed in the supra- 

 plexus, and has been suggested as having some relation to the 

 neuropore. Eycleshymer (10) has described it in Amblystoma 

 larvae up to 14 mm., when the proximal part of the cavity 

 becomes obliterated. Mrs. Gage, however, found it well 

 developed in the adult Diemyctylits, and its cavity, though 

 somewhat constricted, still continuous with the brain cavities. 

 In the Desmognathus it has been more easily seen in frontal, or 



