G30 ANATOMY OF VERTEBRATES. 



of active life. The larva having started into independent ex- 

 istence as a fish, does not relapse into the passive torpor of the 

 ovum, to leave the organising energies to complete their work 

 untroubled by the play of the parts they are to transmute, but 

 step by step each organ is modified, and the behaviour of the 

 animal and its life-sphere are the consequence, not the cause, of 

 the changes. 



The external gills are not dried and shrivelled by exposure to 

 the air, nor does the larva gain its lungs by eiforts to change its 

 element and inhale a new respiratory medium. The beak is 

 shed, the jaws and tongue are developed, and the gut shortened, 

 before the young Frog is in a condition to catch a single fly. 

 The embryo acquires the breathing and locomotive organs — 

 gills and compressed tail — while imprisoned in the ovum ; and 

 the tadpole obtains its lungs and land-limbs while a denizen of 

 the pool : action and reaction between the germ and the gela- 

 tinous atmosphere of the yolk, or between the larva and its 

 aqueous atmosphere, have no part in these transmutations. The 

 Uatrachian is compelled to a new sphere of life by antecedent 

 obliterations, aljsorptions, and developements, in which external 

 influences and internal eftbrts have no share. 



The phenomena of batrachian metamorphosis, that each spring 

 are observable wherever there is a pool of water in a green field 

 of England, are amongst the most suggestive and instructive 

 which the animal economy affords. 



§ 120. Developenient of Scaled Reptiles. — From the difference in 

 tlie structure of the ovum in the scaled and naked Reptiles, the pro- 

 portion of the food-yolk to the germ-yolk is much greater in the 

 former, and the formation of a germ-mass l^y the diffusive process 

 of successive fissions is restricted to a smaller pi-oportion of the 

 ovum than in Fishes. The formation of the embryonic trace closely 

 resembles that in the Fish and Frog ; but, instead of rising above 

 the yolk-ball, the embrj^o sinks into it ; first by the head, which, 

 as it plunges in, gets covered by a fold or hood of the ' serous ' or 

 outer embryonal cell -layer, drawn progressively over the body 

 imtil it is sheathed to beyond the heart ; then the tail, bending 

 down, acquires a caudal sheath ; and the rest of the trunk sinking, 

 the margins of the serous bed are produced over it continuouslj' 

 with the bodies of the cephalic and caudal sheaths, contracting 

 concentrically until the wliole embryo is inclosed in a ' serous ' bag, 

 reflected, as it seems, from the umbilicus, and thus the ' amnios,' 

 fig. 445, a, is constituted. The embryo being imprisoned in the 

 scrum of tliis bag, brancliiie could not act, and arc not dc\elopcd ; 



