30 
MR. W. K. PARKER OK THE STRUCTURE AND 
that the first and second extra-brancliials (Plate 4, fig. 3, ex.br l ~ i .) have lost much of 
their pouch-like shape, and are but little broader than the second and third; they all 
become very narrow bands, and thus vanish, with the exception of a common remnant 
that becomes fused with the permanent hyo-branchial plate. 
The third intra-branchial ( c.br 3 .) is very long, as long indeed as the counterpart 
bars are in Selachians, Ganoids, and Teleostei; in its normal position, round the 
pharynx, this bar just reaches half way up the side, and in those Fishes is surmounted 
by the epi-branchial. 
In the Dactylethra Tadpole (Phil. Trans. 1876, Plate 58, fig. 1) the whole skeleton 
of the branchial pouches is one continuous structure; here the intrabranchials are 
nearly segmented off from the extra-branchials, but there is an isthmus of cartilage 
uniting them in Rana, as may be seen in the section (fig. 4, c.br 1 ., ex.br 1 .). 
I must pass again to Rana pipiens for my next stage ; the reader will, however, 
easily eliminate the non-essential specific variations, at any stage, from those more 
important step by step processes that transform a Cyclostomous skull into one with 
a widely gaping mouth and jaw-hinge far behind, like that we see in certain aberrant 
Selachians— Notidanus and Cestracion. 
Fourth Stage.- —2 (continued). Tadpoles of Rana pipiens (C) with tail lessening and 
all the legs free. 
This stage (Plate 3, figs. 4-13, and Plate 9, figs. 1-6), of this species, shows that 
in the largest and most generalised of the genus the branchial apparatus remains in 
full development and function much longer than in the lesser Frogs. They are two 
or three years (according to Wyman) before they have finished their metamorphosis, 
and that excellent naturalist kept them (in captivity) six or seven years as Tadpoles. 
In R. clamata (Stage B) the nasal roofs just appear as the branchial pouches are 
dwindling away. In R. pipiens the whole nasal labyrinth is perfected before any sign 
of diminution appears in the gill-pouches. 
This indicates that the Tadpole of the Bull Frog is well fitted out with special 
sense organs long before it finishes its metamorphosis, and suggests the probability of 
a still slower metamorphosis in former epochs. Pseudis offers a remarkable confirmation 
of this view. 
Comparing this with the two last stages we shall see both the general progress and 
the specific differences that are found to be even in larvae of the same genus. 
The most striking change is the elongation of the palato-pterygoid band (Plate 3, 
figs. 4-6, p.pg.), thrusting the suspensorium outwards. Then the “waist” of the 
cranial box is narrower, and its sides more chondrified (fig. 6). 
The roof bones are segmented across (fig. 4, f,pf and, contrary to wont, the frontals 
are only half the size of the parietal s. 
Below, the parasphenoid (fig. 5, pa.s.) has become more develojDed, and foliated 
behind ; in front, the membranous layer seen in the earlier larva (fig. 2) is now partly 
