286 



EMBRYOLOGY. 



470. Towards the close of the embryonic period, after the 

 fortieth day, the embryo acquires a more definite shape. The 

 head is more completely separated from the yolk, the jaws 

 protrude, and the nostrils approach nearer and nearer to the 

 end of the snout ; divisions are formed in the fin which sur- 

 rounds the body ; the anterior extremities, which were indi- 

 cated only by small protuberances, assume the shape of fins ; 

 and, finally, the openings of the gills appear, one after the 

 other, so that we cannot now fail to recognize the type of 

 fishes 



471. In this state the young white-fish escapes from the 

 egg about the sixtieth day after it is laid (fig. 309) ; but its 



development is still in- 

 309. complete. The outlines 



are yet too indistinct to 

 indicate the genus and 

 the species to which 

 the fish belongs ; 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 posi- 

 tion 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 com- 

 pletely taken into the animal (461). The duration of these 

 metamorphoses varies extremely in different fishes ; some accom- 

 plish it in the course of a few days, while in others months are 

 required. 



47 2. In frogs, and all the naked reptiles, the development 

 is very similar to that of fishes ; it is somewhat different in the 



Fig. 310. 



Fig. 311. 



