602 AMPHIBIA. 
three branchial arches. The mouth, which has pre- 
viously been merely a blind pit, opens into the gut, 
the gut itself lengthens rapidly, and becomes coiled 
like a watch-spring; the larve feed eagerly on vegetable 
matter and increase in size. The glandular crescent forms . 
two small discs, which gradually disappear as the power 
of locomotion increases. About the time when the mouth 
is opened, four gill clefts open from the pharynx to the 
exterior. 
A second period, the true tadpole stage, now begins. 
A skin-fold or operculum covers the external gills, 
which then atrophy, and are replaced by “internal” 
gills developed on the ventral halves of four branchial 
arches. These gills, though called internal, are covered 
with ectoderm Wke their predecessors, and are com- 
parable not to ordinary fish-gills, but to the external 
gills of Polypterus, Protopterus, and Lepidosiren. The 
mouth acquires horny jaws, and the fleshy lips bear 
horny papilla. By the continued growth of the opercular 
fold the gillchambers are closed, with the exception of 
a single exhalant aperture on the left side. Through 
this opening, the water which is taken in by the mouth 
in respiration passes outwards, having washed the gills’ 
on its way. 
In the third period the rudiments of the limbs appear. 
The fore-limbs are concealed within the gill-chambers, and 
so are not obvious until later; but the hind-legs may be 
watched in the progress of development from small papillz 
to the complete limb. 
The lungs are developed as outgrowths from the ceso- 
phagus, even before hatching, but grow very slowly. 
After the appearance of the hind-legs, the larvee come to 
the surface of the water to breathe, showing that the lungs 
are now to some extent functional. At this stage the 
tadpoles, now about two months old, are at the level of 
Dipnoi. 
The changes in the relations of the blood vessels, which 
accompany the successive changes in the methods of 
respiration, and render these possible, are somewhat com- 
plicated. 
When respiration is by the gills only, the circulation 
