Fig. 123. 



DEVELOPMENT OF THE YOUNG WITHIN THE EGG. 149 



egg, about the sixtieth day after it is laid, (Fig. 123,) but 

 its development is still 

 incomplete. The out- 

 lines are yet too indis- 

 tinct to indicate the 

 genus and the species 

 to which the fish be- 

 longs ; at most we distinguish its order only. The opercula 

 or gill-covers are not formed ; the teeth are wanting ; the 

 fins have as yet no rays ; the mouth is underneath, and it 

 is some time before it assumes its final position at the most 

 projecting point of the head. The remainder of the yolk is 

 suspended from the belly, in the form of a large bladder, but 

 it daily diminishes in size, until it is at length completely taken 

 into the animal, (304.) The duration of these metamorphoses 

 varies extremely in different fishes ; some accomplish it in the 

 course of a few days, while in others, months are required. 



315 a. In frogs and all the naked reptiles, the development is very 

 similar to that of fishes. It is somewhat different in the scaly rep- 

 tiles, (snakes, lizards, and turtles,) which have peculiar membranes 

 surrounding and protecting the embryo during its growth. From 

 one of these envelopes, the allantois, (Fig. 125, ,) is derived their 

 common name of Allantotdian Vertebrates, in opposition to the naked 

 reptiles and fishes, which are called Anattantotdian. 



315 b. The AllantoTdian Vertebrates differ from each other in 

 several essential peculiarities. Among Birds, as well as in the scaly 



Fig. 124. 



Fig. 125. 



reptiles, we find at a certain epoch, when the embryo is already dis- 



13* 



