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 {58}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. [Illustration: FIG. 10.–1, Front view of the mouth of a tadpole of _Rana temporaria_, showing the transverse rows of tiny horny teeth; 2, three successive horny teeth, much magnified. (After Gutzeit.)]