Development and Metamorphosis.—In a great number of batrachians, including most of the European species, the egg is small and the food-yolk is in insufficient quantity to form an external appendage of the embryo. But in a few European and North American species, and in a great many inhabitants of the tropics, the egg is large and a considerable portion of it persists for a long time as a yolk-sac. Although the segmentation is always complete, it is very irregular in these types, some of which make a distinct approach to the meroblastic egg.
With the exception of a number of forms in which the whole development takes place within the egg or in the body of the mother, batrachians undergo metamorphoses, the young passing through a free-swimming, gill-breathing period of considerable duration, during which their appearance, structure, and often their régime, are essentially different from those of the mature form. Even the fossil Stegocephalia underwent metamorphosis, as we know from various larval remains first described as Branchiosaurus. They are less marked or more gradual in the Apoda and Caudata than in Ecaudata, in which the stage known as tadpole is very unlike the frog or toad into which it rather suddenly passes (see [Tadpole]). In the Caudata, external gills (three on each side) persist until the close of the metamorphosis, whilst in the Apoda and Ecaudata they exist only during the earlier periods, being afterwards replaced by internal gills.
Many cases are known in which the young batrachian enters the world in the perfect condition, as in the black salamander of the Alps (Salamandra atra), the cave salamander (Spelerpes fuscus), the caecinan Typhlonectes, and a number of frogs, such as Pipa, Rhinoderma, Hylodes, some Nototrema, Rana opisthodon, &c. A fairly complete bibliographical index to these cases and the most remarkable instances of parental care in tailless batrachians will be found in the interesting articles by Lilian V. Sampson (42), and by G. Brandes and W. Schoenichen (43). It will suffice to indicate here in a synoptic form, as was done by the present writer many years ago, when our knowledge of these wonders of batrachian life was far less advanced than it is now, the principal modes of protection which are resorted to:—
1. Protection by means of nests or nurseries.
A. In enclosures in the water.—Hylafaber.
B. In nests in holes near the water.—Rhacophorus, Leptodactylus.
C. In nests overhanging the water.—Rhacophorus, Chiromantis, Phyllpmedusa.
D. On trees or in moss away from the water.—Rana opisthodon, Hylodes, Hylelia platycephala.
E. In a gelatinous bag in the water.—Phrynixalus, Salamandrella.
2. Direct nursing by the parents.