1 84 AMPHIBIA 



The eggs of an ordinary frog or toad are spherical bodies, 

 surrounded by a thin membrane and one or two gelatinous 

 envelopes, formed during the passage down the oviducts, the 

 outer capsule swelling out in the water, after oviposition. The 

 upper half of the sphere is more or less pigmented, varying from 

 pale brown to black according to the species, and this colora- 

 tion may extend to the whole sphere. The eggs are small, 

 usually one or two millimetres in diameter, containing a com- 

 paratively small amount of nutritive matter for the embryo, 

 and very numerous, numbering several hundreds or even thou- 

 sands. 



After fecundation the eggs undergo a process of division 

 or segmentation, vertical and horizontal furrows appearing 

 over the whole sphere (extreme holoblastic type), resulting 

 in a great number of cells out of which the tissues of the 

 future embryo are formed. The whole egg becomes con- 

 verted into the embryo, and this state of things, which, as 

 we shall see further on, is by no means universal in this 

 class of animals, has been one of the great arguments of 

 the celebrated Spallanzani, in the eighteenth century, in 

 favour of the theory of the pre-existence of the embryo in the 

 unfertilised egg. Such a lack of vitelline food necessitates a 

 very early liberation of the larva, in order that it may provide 

 food for itself, and at this period it is of course exposed to 

 much greater dangers than if turned out into the world in a 

 less embryonic condition. The amount of protection which 

 the gelatinous envelopes afford the embryo varies considerably 

 according to the species. In some, these envelopes soon dis- 

 solve, so as to release the embryos almost before they are able to 

 execute any spontaneous movements ; they, so to say, drop out 

 and become fixed to the outer surface of the remains of the 

 envelope ; whilst in others they develop much further within 

 the egg and become liberated by their own action. 



In the first condition of the larva, the head is large and 

 distinct from the elongate body, the tail absent or rudiment- 

 ary. The head is cleft below by a longitudinal groove, in the 

 middle of which a transverse or rhomboidal depression repre- 

 sents the first rudiments of the mouth ; on each side and in 

 front of this depression, a pit indicates the nostril, and behind 

 it is a grooved, curved or angular transverse fold which 



