Herkick, i\[orp]iology of Nervous System. 163 



epiphysis were traced, with apparent certainty to the supra- 

 commissura. 



Cephalad to tlie habenuljE, as well as immediately above 

 them, the dorsal sac communicates with the third ventricle, 

 the lateral ventricles being entirely distinct. Slightly further 

 cephalad the dorsal sac, is shut off from a median portion 

 which must be considered as homologous with the aula. 

 Into this median portion the cavities of the lateral ventricles 

 open, as usual, the only difierence being that the aula is 

 more elongated than usual, and the portse are correspondingly 

 enlarged. It has sufficiently appeared from the above that 

 the dorsal sac is not a portion of the cavity of the pros- 

 encephalon, but of the diencephalon. It is not a new struc- 

 ture, but one found in reptilia as well, and pertains to the 

 epiphysis. 



In Scaphirhynchus the relations are essentially the same, 

 though the plexiform development along the median wall is 

 much greater, and the epiphysis is scarcely as large. The 

 caudal portion of the dorsal sac is smaller, and an irregu- 

 larity is introduced by the greater size of the right habena. 

 The less retro-development of the hemisphere may ex- 

 plain why the epiphysis is not arched so far caudad as in 

 Lepidosteus. 



Serious exception niust be taken to the use of the term 

 "falx" for the median projection of the pallium or dorsal 

 roof of the cerebrum. There can be no reason for rejecting 

 the homology between the membranous roof of the cerebrum 

 in fishes and the mantle part of the cerebrum. The extent to 

 which cellular elements develop in the walls of the embryonic 

 nerve tube varies within the broadest limits. The birds have 

 a great reduction in the cellular mantle structures. The 

 median portion of the pallium must be homologous with the 

 median or interventricular portion of the mantle in other 

 vertebrates. The portion of the investing membranes, which 

 is associated with this fold, must represent the falx. 



Histology of the Brain oj Lepidosteus and Scaphirhyn- 



