534 
MR. W. K. PARKER ON THE STRUCTURE AND DEVELOPMENT 
Behind the hyoid arch, and partly hidden under its opercular fold, there are, on each 
side, three dermal vegetations ; the first of these is lowest in position, and the last is 
highest. These are the rudimentary “ external branchise ; ” they are decurved, claw- 
shaped buds, the terminal claw of which is ensheathed in a thick base, which itself 
gives off two smaller buds externally : thus we have here the first three of the branchial 
filaments. 
If we look at the ventral surface of the throat (fig. 3, cl. 3-5) we can see three gaping 
spaces where the skin has undergone dehiscence, and these correspond to the fossae seen 
on the inner side of the pharynx (fig. 4, cl. 2-5) ; these clefts are lined on the inside 
with hypoblast, and on the outside with epiblast, these two laminae meeting here at 
these spaces, as in the early “ blastopore.” The first cleft (figs. 1 & 4, cl. 1) is a very 
small, roundish, thin space, which, as far as I can see, never becomes quite open either 
in Urodeles or Anura. This cleft is the first to appear in the Shark according to 
Mr. Balfour (Develop, of Elasmobranch Fishes, p. 539), who shows it in his stage “ G.” 
“ The alimentary canal in the region of the head exhibits on each side a slight bulging 
outwards, the rudiment of the first visceral cleft. This is represented in the figure 
(plate 24. fig. G) by two lines (fig. 1, v.c.). The visceral clefts at this stage [a stage 
corresponding to the one next before my first] consist of a pair of simple diverticula 
from the alimentary canal, and there is no communication between the throat and the 
exterior.” 
The sectional view of the Axolotl embryo shows this diverticulum with three or four 
behind it ; it is a temporary structure in the Urodele, but becomes the tympano- 
Eustachian cavity in the Batrachia, and is persistently open outside in the Elasmobranchs 
as the “ spiracle the “ annulus ” of the Frog is the counterpart of the spiracular 
cartilage of an Elasmobranch, which is generally a single “ visceral ray,” corresponding 
to the numerous rays on the branchial arches*. 
The hyoid and mandibular arches are nearly of a size, and they both project forwards 
where the two halves meet below ; the latter projects most, and in front of this “ chin ” 
there is a transverse fossa, somewhat in the form, as to outline, of an hourglass. The 
section (fig. 4, m ) shows that dehiscence of the ventral wall has taken place to some 
degree, and the naso-frontal process forms a rest ” for the mentum. If the lips are 
parting, the mouth-cavity is still considerably occluded by a mass of cells, that, in this 
* In Gegehbaur’s work on the skull of the Elasmobranchs, tko “spiracular cartilage” is shown to be single 
as a rule; in Scymnus (plate 11. fig. 1, Ter.) there are two, and in Centrophorus calceus (plate 12. fig. 1, hr.) 
there are three. Now my own drawings show four short external branchial filaments in the spiracle of an 
embryo of Scyllium canicula (which was nearly an inch in length) ; there can be therefore no difficulty about 
the homology of the ray or rays growing from that arch as tending to form an operculum to these temporary 
gills. 
In the Batrachia, where the first cleft is much more developed than in the Urodela, although it does not open 
externally, this mandibular branchial ray, or opercular cartilage, reappears, becoming first crescentic and 
then circular, and serves for an “ annulus tympanicus ” in these tribes, whose external auditory apparatus 
becomes on a sudden, as it were, so highly developed. 
