290 Susanna Phelps Gage 



Wilder (55, Fig. 471 1, 57, Fig. 25) there is a mesal pocket 

 of endyma which is reflected from the supracommi.ssure over 

 the cephalic aspect of the conarium. Cephalad it is contin- 

 uous with the endyma covering the plexuses which lie in the 

 elongated interval between this point and the porta. Whether 

 this sac with its cephalic extension to the porta, and its 

 intimate relation to the large vessels and plexuses which lie 

 dorsad of it can be identified with the paraphysis of amphibia 

 is not known but certainly a strong resemblance to the facts in 

 diemyctylus can be seen. As the roof of the cavities in this 

 region is a mere membrane it does not seem improbable that a 

 structure, in lower forms closer to the portse, might be drawn, 

 with the great vessels with which it is associated, to a distant 

 point by the growth of the callosum, around the caudal end of 

 which those great vessels effect their entrance to the brain 

 plexuses. 



Burckhardt (7, p. 398) suggests that the caudal border of the 

 supraplexus rather than the supracommissure be considered 

 as the boundary between the prosen-and diencephal. From 

 the preceding studies it would appear that the opening of the 

 paraphysis would be a more exact demarcation in the groups 

 in which it has been identified especially if the form found in 

 amphibia be considered from its exact definiteness, the typical 

 condition. The embryonic form (Fig. 73) with the open- 

 ing of the paraphysis in a partition between the two segments 

 would be the point of departure, on the one hand, toward 

 those forms in which the segments are not divided by a parti- 

 tion and which have no plexuses, on the other toward those 

 in which the plexuses are well developed and the segments 

 distinct. 



In fishes this would be a convenient landmark, as in am- 

 phibia. In lamprey the part in which the left habena lies 

 must be conceded to belong to the diencephal hence the ex- 

 treme cephalic position ot the part here called paraphysis need 

 not be a bar to considering its opening as the dorsal limit be- 

 tween the prosen-and diencephal. Among the mammals, 

 should the inference made above as to the paraphysis be cor- 

 rect, the case is more difficult because much, or perhaps all of 



