α. Round the legs; the young leaves the egg in the tadpole stage: Alytes.
β. In the enlarged vocal sacs; the young leave in the perfect state: Rhinoderma.
b. By the female:–
α. Attached to the belly: Rhacophorus reticulatus.
β. Attached to the back; the young complete their metamorphosis within the egg: Pipa.
γ. In a dorsal pouch which the young leave as tadpoles: Nototrema marsupiatum;–or in the perfect state: Nototrema testudineum, N. cornutum, N. oviferum, N. fissipes, and Hyla goeldii.
The development and metamorphosis of many species have been described in the systematic part. The following is a short general account of some of the more important features. Metamorphosis in the Apoda and Urodela is restricted chiefly to the reduction of the gills, the closing of the clefts, and the loss of the gill-chamber and the finny margins of the tail; but the change from the tadpole to the final Anurous animal implies an almost entire reorganisation.
Fig. 9.–Four stages of the development of the adhesive apparatus (A) of Bufo vulgaris; M, Mouth; Sp.T. spiracular tube. In 3 the gills are almost completely hidden by the united right and left opercular folds. The small outlined figures indicate the shape and natural size of the tadpoles. (After Thiele.)
In the earliest condition the embryo consists of a large head and body, while the tail is still absent. Behind the beginnings of the future mouth appears a transverse crescentic fold, with the convexity looking backwards, which develops into the paired or unpaired adhesive apparatus. This consists of large complex glands, developed in the Malpighian layer, originally covered by the cuticula, which soon disappears, whereupon the sticky secretion enables the larva to attach itself to the gelatinous mantle of the egg, later on to weeds or other objects in the water. The name of suckers, often applied to this apparatus, conveys a wrong idea, there being neither muscles nor any suctorial function. The shape of this organ undergoes many changes during the early life of the individual, and differs much in the various genera, affording thereby diagnostic characters.[[29]] At first a crescent, it divides into a right and a left oval or disc, which either remain asunder and behind the mouth (Rana, Bufo), or they move forwards to the corners of the mouth (Hyla) or further back, and unite again more or less completely, as in Discoglossus and Bombinator. It is mostly of short duration, and disappears by the time that the larva, by the proper development of the gills and the tail and the functional mouth, changes into the tadpole. But in a few species these discs transform themselves into an elaborate ventral disc. Such an organ persists throughout the greater part of the tadpole-stage in certain Oriental species of Rana, all of which, when adult, possess fully webbed toes and strongly dilated discs on the fingers and toes, e.g. Rana whiteheadi, R. natatrix, and R. cavitympanum of Borneo, R. jerboa of Java (this larva was originally described and figured as that of Rhacophorus reinwardti), and R. afghana of the Himalayan system. These tadpoles, at least those of R. jerboa, are further remarkable for having the "spiracular" opening very far back on the left side, nearer to the base of the tail than to the snout, so as to be well out of the way when the creature has attached itself by the adhesive disc.