RANA. 357 
The step to the newly-hatched tadpole is not great. 
The exterior shows the long tail already formed, the developing eyes 
and ears on the head, the two pair of external gills (soon followed by a 
third), and, on the ventral surface of the head, a large sucker The 
openings of the stomodeum and anus are also seen. The former is 
blind, but the latter is already an aperture leading into the gut. In 
longitudinal section we may notice the commencing liver as a ventral 
diverticulum of the gut, and in front of it the simple tubular heart. 
The pronephros is present as three ciliated funnels on each side, leading 
by a paired archinephric duct on each side to the hind-end of the gut. 
Fig. 255.—YouNG TADPOLE DISSECTED FROM THE VENTRAL 
SIDE. 
(Mainly after MarsHact.) 
Mouth. 
Afferent 
A Branchials, 
wi 5 ity. 
Opercular INC | taut 
Cavity.: \ } : 
creak ercular Aperture. 
eart. Glomerulus. 
Pronephros. a 
; : Mesonephros. 
Pronephric Duct. : 
Aorta, vy) — Intestine. 
Hind-Limb. 
Cloacal 
Aperture. 
A day or two after hatching the mouth opens,” with horny jaws; the 
yolk has been used up and the tadpole feeds upon small water-plants. 
At the same time the four gill-clefts open and internal gills are formed 
on their walls. The external gills then atrophy. The gill-slits be- 
come covered over. by a fold of skin or ofercat/um on each side. A 
small opercular aperture remains on the left side, but none on the right. 
.The tail is provided with a dorsal and ventral median fin, and the tadpole 
swims actively by its action. During this stage, whith lasts for about 
six weeks from hatching, the tadpole has an internal organisation like 
a fish. The two-chambered heart drives blood by afferent: branchials 
‘to the four gills. The limbs developas small buds, the fore-limbs- in 
the opercular chamber and the hind-limbs beside the anus. The 
mesonephros arises as a set of small tubules which join the pronephric 
duct and gradually replace the pronephros. The lungs then develop 
and become functional. i 
The tadpoles frequently come to the surface and take air into the 
lungs. Thus is instituted a stage comparable to the Dzpzoz, in which 
both forms of breathing are functional. At about two months this 
