The Brain of Diemydylus Viridescens 297 



not circumscribed by homologous parts in the two types. The 

 term paracoele however, applying to the space between the 

 cerebrum and pallium in fishes is perfectly comparable to the 

 paracoele of other forms. 



RHINENCEPHAL. 



In this article the term rhinencephal has been used as 

 though the olfactory region might be considered a segment as 

 independent as the prosencephal. The lack of a distinct 

 mesal portion in higher forms has led Wilder (56 p. 114) to 

 reject such independence. 



Upon embryological grounds it seems as though the rhinen- 

 cephal were equally entitled with the prosencephal to a share 

 of the aula as a mesal cavity. In the embryo there are two 

 portions of the forebrain — one associated with the developing 

 olfactory nerves, the other lying next the diencephal — with a 

 large common cavity. In the larval diemyctylus (Fig. 71), 

 a cephalic part of the cavity belongs to the olfactory region, a 

 caudal to the cerebrum ; the porta gives free opening of both 

 into the aula. With the growth of the callosum and callosal 

 eminence, the olfactory lobes are pushed away from their evi- 

 dent connection with the aula by that which may be called an 

 intercalated portion of the cerebrum ; the caudal part of the 

 cerebrum retains its original relations (Fig. 36). 



The brain of the lamprey (Fig. 103), does not progress be- 

 yond the condition of the larval diemyctylus. From the 

 porta the cavity forks, the cephalic part extending into the 

 olfactory lobe (Fig. 104), the caudal, a short distance into the 

 cerebrum proper (Fig. 106). 



From sections of amia (Fig. 93-95) it is seen that on the 

 ventral side it is difficult to set a caudal limit to the olfactory 

 lobe which may extend quite to the precommissure. The pros- 

 encephal would then be represented by a wedge with a nar- 

 row base, both segments having equal share in the large aula. 



In adult mammals the original conditions are masked by 

 the great growth of the callosum and fornix but in early 

 embryos the relations are simple and not unlike that of the 



