DEVELOPMENT OF THE SKULL OF THE COMMON FROG. 
145 
hinder part of which the ear-sac is planted, there is a considerable space beneath the 
derm, which is filled now, and for some weeks to come, with gelatinous stroma. 
Second Stage. — Frog-tadpoles 4 lines long. 
If figs. 2 & 10 be compared, it will be seen what great advances have been made in 
the development of the larval Frog, although the size is only one line longer, and the 
yelk-mass still fills the greater part of the thoracico-abdominal cavity. 
The fore part of an embryo at this stage presents the appearance shown in fig. 10 
when seen laterally, and in fig. 11 when viewed from below. 
The lips (Ip.) and mouth (m.) are now well developed, and the head is less bent upon 
itself ; but the change which is of intensest interest to the morphologist is the comple- 
tion of the visceral clefts (1, 2, 3, 4, 5, 6). 
This dehiscence into free bands of the cephalic visceral walls characterizes the Verte- 
brate animal, and wholly distinguishes the stomo-pharyngeal series from all other arches. 
In one sense the mouth itself is a cleft in the lowest part of the visceral wall between 
the preoral and first postoral arches ; but I would rather consider it comparable to a 
neural “ fontanelle,” and therefore as an imperfect closure of the “ membrana reuniens 
inferior,” than to secondary openings (by dehiscence), such as we have seen the true 
clefts to be*. The opening between the first and second postoral arches (figs. 10 & 11, 1) 
only appears on the outside, in the lower third, and that imperfectly, for the skin is 
rather greatly attenuated than completely cleft ; this is the rudiment of the “ tympano- 
eustachian ” passage. A perfect cleft, however, does appear between the preoral and 
1st postoral arch ; at present it can only be seen from the inner side (fig. 13, i.n.) : this 
is the first appearance of the “ inner nostril and the cleft is only completed when the 
nasal tube is perfect. This takes place when the central depressed skin over the nasal 
sac (ol.) has grown inwards, and coalesced with the mucous membrane lining the inner 
edges of the cleft (fig. 13, i.n.). 
Only for a short time, and at this stage, can the cleft between the first and second post- 
oral arches be seen externally (figs. 10 & 11, 1) ; it is lozenge-shaped, and has the same 
direction as its more perfect successors ; and although the skin becomes greatly attenu- 
ated at this part, I could not discover a perfect passage. It soon closes up, afterwards 
to reappear as the “ membrana tympani,” with its overlying thin dermal layer. 
The gill-buds have now become compound papilla;, of a palmate form ; and the first 
(fig. 10, hr. 1) has eight secondary papillae upon it. A retral fold of the skin over the 
second postoral arch covers the cleft between itself and the first branchial on the out- 
side; this is the “operculum” (figs. 10 & 11, op.). 
The branchial clefts (2-6) are about half as long as the depth of the embryo at the 
same part, and are equidistant from the dorsal and ventral line ; they are rounded above 
and more acute below ; the last is the most patulous ; and behind it the thoracic wall is 
incurved. The mouth (fig. 11, m.) is beginning to take on the rounded form proper to 
* The lozenge-shaped opening (Plate III. fig. 1, m .) may be due to the absorption of a layer of cutaneous cells, 
MDCCCLXXI. 5 
