78 INTRODUCTION. 



XI. Eggs. 



The eggs are spherical vitelline bodies, surrounded 

 by a thin, elastic, cortical membrane or chorion and 

 one or two gelatinous envelops, formed during their 

 passage down the oviducts : the outer capsule swells 

 out in the water after ovi position, and varies accord- 

 ing to the families or genera; the inner is absent in 

 the Pelobatidae. With the exception of Alytes, the 

 upper pole of the vitelline sphere is always pigmented, 

 varying from pale brown to black, and this coloration 

 may extend on the whole sphere, — as, for instance, in 

 Bufo vulgaris, or on the whole save a small whitish 

 spot at the lower pole, as in liana temporaria. The 

 inner mucilaginous envelop is formed in the upper 

 portion of the oviduct, and surrounds each ovum ; the 

 outer, which has been termed " adhesive envelop," is 

 formed low r er down the oviduct, and either surrounds 

 each single ovum, whether free or agglutinated in 

 masses, or forms a common investment in which the 

 ova are disposed irregularly or in files. 



All eggs, except those of Alytes, are deposited in 

 the water. If laid singly, each is fixed by its adhesive 

 envelop to submerged bodies ; if in strings or bands, 

 these are twined round plants. Egg-masses are usually 

 also fixed to plants, except those of Rana temporaria, 

 which are endowed with a peculiar buoyancy, and 

 simply float on the surface. 



The eggs of some Batrachians have a strong odour 

 of fish, this being most noticeable, among European 

 forms, in Pelobates fuscus. 



The amount of protection which the mucilaginous 

 envelops afford the embryo varies considerably. In 

 Discoglossus, Pelodytes, Pelobates, and Bufo they soon 

 partly dissolve, so as to release the embryos almost 

 before these are able to execute any spontaneous move- 

 ments ; the embryos, so to say, drop out and become 

 fixed by their adhesive subbuccal apparatus to the 



