272 Susanna Phelps Gage 



Mesencephal. — This differs considerably from the adult. 

 From the dorsal aspect (Fig. 64) a broad band of cinerea is 

 visible, while in the adult only a very limited area at the cau- 

 dal end is seen (Fig. 4). This is part of the ental cinerea 

 which has not yet been covered by the growth of alba. The 

 cavity also differs in form, since at the caudal end a very wide 

 lateral expansion exists (Fig. 84) which later entirely disap- 

 pears (Fig. 26). This early character resembles the post-optic 

 expansion of the frog, reptiles and birds. 



The middle and cephalic portion of the cavity (Fig. 81- 

 83), shows a narrow extension to the roof The extent of 

 this narrow portion is shown by the more lightly shaded por- 

 tion of the mesocoele in figure 67. The roof in this part con- 

 sists of a single layer of endymal cells, but it is evident that 

 the closely approximated sides are in process of union. This 

 process is complete in the adult, a mere trace of the past his- 

 tory being retained in the scattered cells which hold their 

 place in the alba (Fig. 26). 



Epencephal. — The cerebellum upon the meson is very small 

 (Fig. 67) and shows only a trace of alba (Fig. 84), yet lat- 

 erad it is larger (Fig. 83). The lateral reces.ses of the epicoele 

 extend far cephalad. The relations of the cavity and the 5th 

 nerve vShown in figure 82 seem to confirm the conclusion that 

 this cavity with its lateral recesses must be assigned to this 

 segment rather than to the metencephal, though the relations 

 are more obscure in the adult. The lateral wing of cinerea 

 seen (Fig. 25) in the adult apparently indicates the partial 

 closure of the cephalic portion of the epicoele. Figures by 

 Herrick (24) of the sturgeon's brain in this region show clear- 

 ly the relations which in amphibia are very obscure, and are 

 confirmatory of the above conclusion. 



Metencephal. — From the dorsal side, this segment shows a 

 broad band of cinerea (Fig. 64) upon either side of the plexus. 

 This almost disappears in the adult (Fig. 4). 



The metaplexus is wholly undifferentiated being formed of 

 a simple layer of cells not much different from the other endy- 

 mal cells, and over it a few mesodermal cells which probably 

 belong to the pia (Fig. 86-88). There is no indication of a 

 metapore. 



