INTRODUCTION 195 



affords the embryo varies considerably, as In some cases 

 they partly dissolve In order to release the young tadpoles 

 before the latter are able to make any movement, whilst 

 in others a considerable amount of wriggling on the part 

 of the embryo Is necessitated to ensure its release. The 

 size of the egg or of the larva stands in no relation to that 

 of the parent. The number of eggs laid varies also con- 

 siderably, Bufo viridis, for Instance, laying up to 12,000, 

 while a brood of Alytes may contain only twenty. The 

 following is a list of most of the European Frogs, giving 

 approximately the maximum number of the eggs deposited 

 by each species — 



Bufo viridis about 11,000, Rana esculenta about 10,000, 

 Bujo vulgaris about 6,000, Bujo calamita about 3,500, 

 Rana temporaria about 3,000, Pelobates fuscus about 2,000, 

 Rana arvalis about 2,000, Pelodytes punctatus about 1,500, 

 Rana agilis about 1,000, Hyla arborea about 900, Bonibi- 

 nator pachypus about 300, Alytes obstetricans about 200. 



In most newts, and In many frogs and toads, a period 

 of at least a fortnight elapses before the young emerge ; 

 in Discoglossus pictus, however, thirty hours suffice. 



On emerging from the egg the embryo has a very large 

 head and body. In a frog the external gills and the long, 

 compressed tail are only feebly developed when the tadpole 

 is just hatched, while the mouth is provided with a much 

 developed adhesive apparatus, by means of which the 

 young attach themselves to plants or other objects. The 

 " holder " appears as a crescentic groove very early, before, 

 in fact, any other organ has formed, and disappears, after 

 having undergone various changes, in the first stages of 

 the tadpole period ; Its fullest development coincides with 



