118 EMBRYOLOGY. 



becomes free, and the young animal moves it in violent 

 jerks. 



313. The embryo, although still enclosed in the egg, now 

 unites all the essential conditions for the exercise of the 

 functions of animal life. It has a brain, an intestine, 

 a pulsating heart and circulating blood, and it moves 

 its tail spontaneously. But the forms of the organs are 

 not yet complete ; nor have they yet acquired the pre- 

 cise shape that characterizes the class, the family, the 

 genus and the species. The young White-fish is as yet 

 only a vertebrate animal in general, and except for 

 the fin that surrounds its body, might be taken for the 

 embryo of a frog. 



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

 fortieth day, the embryo acquires a more proper 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 surrounds the body ; the anterior extremities, 

 which were indicated only by a small protuberance, 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. 



315. In this state, the young white-fish escapes from the 



egg, about the sixtieth 

 day after it is laid (Fig. 

 123). But its develop- 

 ment is still incom- 

 Fig. 123. plete. The outlines are 



yet too indistinct for us to recognize 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 as- 



