LIFE-HISTORY OF XHNOPUS LAVIS, DAUD. 811 
quiescent. A strong continuous current suiicient to sway them 45° from the normal 
will not induce more than one in twenty or thereabouts to detach itself. 
The most obvious changes during the first days of this phase are the darkening 
of the pigmentation both of the eye and of the nasal pit, which both become more 
conspicuous, the fin becoming wider, but only very gradually (cf. fig. 22a), and 
three short unbranched external gills appearing on the three branchial arches. The 
gill slits are closed. As the yolk is absorbed the larva becomes more and more 
transparent, and at the same time the ciliation disappears part passu with the 
yolk, except the ciliation in the nasal pits, which persists. As it becomes more 
transparent the larva becomes more and more restless, and about four to six days 
after hatching spends more time swimming about than hanging on by its cement 
organ. At this time the mouth opens and the branchial current of water is set 
up with its rhythmic action. By this time the operculum has grown back from 
the hyoidean arch and fused in the mid-ventral region with the body-wall under 
the pericardium to form the gill-chamber. The upper, lateral ends of the folds 
remain free and form the ventral or outer lip of the spiracle on each side of the 
neck. The spiracle at all stages opens upwards and backwards and is not produced 
into a spout; the inner wall of the spiracle is the body-wall of the animal. 
The mouth opens and the branchial respiratory current begins some hours, 
probably about twelve hours, before the animal takes in food. It is easy to see that 
the gut still contains a mass of yolk, and in sections the cesophagus is found to be 
solid. Hxactly the same condition of things has been described in the common frog 
(MarsHaLt and Buss, 90, pp. 223-4), with the difference that in Rana the branchial 
chamber is not completely formed and that feeding begins after a much longer interval. 
Drawings of the animal at this stage are given in figs. 22 and 22a. In the 
front view of the head an attempt has been made to show the commencing transparency 
of the tissues. It brings out clearly the fact’that the chromatophores of the skin 
are confined to the dorsal surface; the ventral is free from them except in one 
place to be referred to later. Covering the whole underside of the head there is 
a large continuous lymph space, well marked off from the other lymph spaces, which 
I propose to call the suwbmental lymph sac. Through its translucent walls can be 
seen the ventral wall of the buccal cavity. The cement organ is at the extreme 
tip of the head (see fig. 22a), and immediately above it is the mouth or rather 
the lower jaw, still showing at the symphysis the junction of the mandibular arches. 
At this stage the lower lip protrudes in front of the upper; this condition becomes 
still more pronounced in older tadpoles, where the mouth opening comes to lie on 
the upper surface. The ventral chromatophores on the skin of the mandible then 
face upwards and become practically dorsal. This disposition of the mouth vanishes 
at the metamorphosis and is apparently adapted to the peculiar feeding habits of 
the tadpole. The oral tentacles have not yet made their appearance. Above the 
mouth are the large shallow depressions of the nasal pits, with their well-marked 
