282 Susanna Phelps Gage 



existing in the early human embryo, and also found in the 

 lower mammals. Transitional forms must he looked for be- 

 tween the urodele and anurous conditions probably in em- 

 bryonic forms. 



In the larval diemyctylus the position of these commissures 

 (Fig. 6, 91) even more than in the adult (Fig. 6) differs from 

 the frog in relation to the terma. In this and other urodeles 

 and in fishes (Fig. 93) the commissures are at the caudal 

 boundary of the large aula, the terma not rising directly from 

 them as in higher forms to the dorsal part of the portas, but 

 again dipping ventrad and curving around the large portae. 

 This is a modification of the embr3'onic condition in which 

 the aula extends, as the common cavity of the cerebrum, to 

 the cephalic extremity of the brain. It is as though the pro- 

 gress of the caudal development of the mesal walls of the cere- 

 brum, carrying the terma with them, were arrested in the 

 urodeles. Evidence of such a history is found in the larva 

 and the adult in the cinerea which reaches the mesal surface, 

 cephalad of the terma. In the young larva this consists 

 (Fig. 67, 76) of a single layer of endyma, which before the 

 end of larval life becomes several laj'ered and in the adult 

 through a large part of its extent is a mass of scattered cells 

 (Fig. 6, 15) reaching to the mesal surface. 



In fishes the lateral and ventral curvature of the walls of 

 the cerebrum (p. 295) introduces another element of differ- 

 ence. In amia no commissure was found cephalad of the 

 compound one {cm. Fig. 93, 97) representing the callosum 

 and precommisure. Herrick (22, 24), however, in certain 

 teleosts and possibly in lepidosteus has found such a cephalic 

 commissure which he believes to be the callosum. Ahlborn 

 (i) shows a commissure which connects the olfactory lobes 

 directly, the precommissure (Fig. 103, 104). In the present 

 study of the lamprey brain, another band of alba in a position 

 easily overlooked, as it lies ventrad of a deep projection, was 

 found (Fig. 103, 105, rw.). This is more comparable in posi- 

 tion with the commissures of amia and diemyctylus than is 

 the precommissure, but it connects parts which seem homo- 

 logous with the striatums rather than with the callosal emi- 



