LIFE-HISTORY OF XENOPUS LAVIS, DAUD. 817 
(fig. 20) and one killed during the night (fig. 21). The pale brown network in fig. 20 
suggests, by the sharp double outlines occurring in many places, the presence of inter- 
cellular passages into which the chromatophores expand radially and leave pigment 
granules behind adhering to the walls of the passages. 
The Fove-limb.—In fig. 24 the fore-limb is seen lying under a transparent patch of 
integument, and in fig. 25 this region is shown magnified in a slightly older larva. 
Here the arm has burst through the thin wall of the sac, the edges of which are still 
present, and it can be seen that the wall of the branchial chamber immediately in front 
of the arm-sac is intact, nor is the spiracle affected in any way. This is probably what 
PARKER was referring to when he stated, ‘“‘ The fore-limbs are not hidden beneath the 
opercular fold” (ParKeER, ’76, p. 626). The explanation of the difference between 
Xenopus and the more familiar tadpoles is that in the latter the fore-limb develops 
a diverticulum of the gill-chamber which remains in communication with it, so that the 
developing arm protrudes into the branchial space ; i Xenopus a similar diverticulum 
is formed, which becomes completely shut off from the gill-chamber, and the arm cannot 
encroach on that space. The arm emerges in the common frog by breaking through the 
wall of the branchial chamber on the right side and by passing through the spiracle on 
the left side, blocking up this passage completely. The appearance of the arms there- 
fore in typical tadpoles sharply marks the abrupt cessation of branchial respiration. In 
Xenopus the arm appears by the rupture of what may be called the “ brachial sac.” 
This event in no way interferes with the habits of the tadpole. It remains floating in 
mid-water in the same position as before, taking in water at the mouth and passing it. 
out by the spiracles, these being, as shown in fig. 25, Plate IV, quite unaffected by the 
protrusion of the arms. In fact the branchial current is used here not for respiration, 
but for nutrition, and is not interfered with during the metamorphosis. The main 
part of the change into the adult condition is very gradual, and feeding can be 
continued almost without a break while it is goimg on. 
Metamorphosis.—The metamorphosis is completed ten to twelve weeks after 
fertilisation, in a constant temperature of 20° to 22° C. At this temperature the 
whole change from the mature tadpole into the tailless frog is passed through 
in about 15-20 days, but I have no doubt that in its native African pools the 
temperature in the late spring will be much higher’ and the metamorphosis 
correspondingly much more rapid. The commencement of the metamorphosis is 
well marked by the protrusion of the arms, the beginning of co-ordinated swimming 
movements of the legs, and the first appearance of blood-vessels across the width 
of the fin-fold of the tail. It is a remarkable fact that until this time the whole 
of the fin-fold is completely non-vascular. The usual sub-vertebral vessels are of 
course present and supply the tail-myotomes, but they give off, until this period 
of development, no vessels into the fin. The space between the two bounding 
layers of integument is filled with a loose trabecular tissue composed of stellate 
mesenchyme cells, the interstices of which seem to be filled either with fluid or 
