EARLY DEVELOPMENT OF RANA TEMPORARIA. 137 
temporaria, to show the complete separation of the Gasserian ganglia from 
the epiblast. 
Fie. 21.—Transverse section through the head of an Elasmobranch embryo, 
to show the early origin of the fifth nerve. fap 
Fics. 22—24.—Transverse section through the head of a somewhat older 
embryo, to show the fusion of the fifth nerve with the epiblast and the forma- 
tion at this point of the Gasserian ganglion. 
The Figs. 21—24 are drawn from sections kindly lent to me by Dr. Beard. 
Fig. 25.—From Kowalevsky. Transverse section through a developing 
Chiton polii, to show the formation of the nerve-cords by proliferation. 
