EARLY DEVELOPMENT OF RANA TEMPORARIA. 131 
article one or two points of interest derived from the study of 
them, and which are of some importance in connection with 
the development of the cranial nerves. 
They are, moreover, conclusions at which Dr. Beard has 
himself also arrived. 
Fig. 21 represents a transverse section through the head of 
an Elasmobranch embryo, and, owing to cranial flexure, the 
three divisions of the brain are all cut through. Arising from 
the dorsal surface of the hind brain may be seen the two rudi- 
ments which will give rise in development to the trigeminal 
nerve. These arise apparently quite independently of the 
epiblast, with which, at this early stage, they have no connec- 
tion, but are connected solely with the nervous matter of the 
neural canal. 
Figs. 22, 23, and 24, however, are similar sections through 
the head of a somewhat older embryo, the first-mentioned 
being the most anterior section, and the other two being cut 
at slight intervals posterior to this. In these three is seen 
part of the course of the trigeminal nerve, which at one point 
has become clearly fused with the epiblast. It is at this point, 
after the fusion has taken place, that the Gasserian ganglion is 
formed. 
In the sections the cells of the epiblast at this point seem to 
have undergone considerable proliferation, and an examination 
leads to the conclusion that the Gasserian ganglion is, at all 
events in part, formed directly from the epiblast. 
The importance of this origin and relation to the epiblast of 
the Gasserian ganglion, and the fact that the same development 
takes place in the case of the ganglia of the third and seventh 
nerves—in that of the ciliary ganglion the development is par- 
ticularly clear—becomes still more important when taken in 
connection with the particular part of the epiblast with which 
the ganglia are so intimately connected in early stages. 
The ganglia arise along the level of the lateral line 
continued on to the head. 
Professor Marshall has previously described! the ciliary and 
1 “Qn the Head Cavities and associated Nerves of Elasmobranchs,” 
