OF THE SKULL IN THE URODELOUS AMPHIBIA. 
537 
lessens; the Urodela, however, go through less of this change of direction than the 
Batrachia *. 
The first two postoral bars, with their enfolding skin, help to make the head very 
large relatively ; and the opercular vallance {op.) is now very large, and conceals the 
lesser or branchial arches to a great degree. 
Each of these gill-arches has one gill (one on each side), and these single gills sprout 
out from the arch higher and higher up. 
The opercular fold receives two lesser plaits laterally ; for the first gill-arch partly 
overlaps the second with its gill, and this in turn overlaps the third arch with its 
gill ; the fourth arch is barren. 
These three-knobbed, sprouting gills now most resemble the hand of the Aye-Aye 
( Cheiromys ), save that they only bear three fingers. This tridigitate condition is not 
long kept ; for the appearance of new buds will result in the production of an elegant 
branchial feather, of great length ; but no new suckers will grow out from the outer 
edge of the arch, and none appear further in. 
The embryos, at this stage, are becoming lively, and soon acquire considerable 
consistence of their skeletal structure, with their fairly differentiated muscular segments : 
chondrification has begun. 
But the youngest embryo in which it has been possible to dissect and display the 
nascent skeletal structures was half a line longer than the one whose portrait I have 
given and described (Plate 21. figs. 5-7). This larger embryo was 4^ lines long 
(Plate 22. figs. 1, 2). Professor Huxley’s illustration (Proc. Zool. Soc. 1874, plate xxxi. 
fig. 1) is from the study of Axolotls one line longer (5^), and corresponds with my 
next {third) stage f. 
The results given in the two figures (Plate 22. figs. 1, 2) were obtained from an embryo 
that had been hardened in absolute alcohol. It was scalped, washed, cleared of all 
loose tissue, especially brain-cells, stained with carmine, and mounted in glycerine. 
The free mandible is given in both figures, but the other arches only in fig. 2, which 
was drawn as an upper view also, from an object made by slicing away the whole of the 
lower face. These arches must be conceived of as existing in the object shown in fig. 1, 
but hidden beneath the basicranial structures. 
This is the simplest and most fundamental vertebrate skull I have been able as yet 
to demonstrate and depict ; nor have I seen any thing figured by other labourers that 
displays so many of the unopened buds, so to speak, that afterwards expand and grow 
into the complex cranium and face. 
Only the inferior arches are chondrified ; the corpuscles of the cartilage are 
* In that group the mandibular pier has at first its direction downwards and backwards, as in th q first 
stage of the Axolotl ; it soon becomes nearly horizontal , and then, during metamorphosis, swings backwards 
again, regaining, in the adult, the direction it had during the existence of the mesocephalic flexure. 
t They were bred by me, and we both worked from the same source. The smallest I supplied my friend 
with were nearly half an inch long. 
MDCCCLXXVII. 4 G 
