148 EMBRYOLOGY. 



The heart shows itself about the same time, under the form 

 of a simple cavity, (Fig. 121, /,) in the midst of a mass 

 of cells belonging to the middle or vascular layer. As soon 

 as the cavity of the heart is closed in, regular motions of 

 contraction and expansion are perceived, and the globules 

 of blood are seen to rise and fall in conformity with these 

 motions. 



312. There is as yet, however, no circulation. It is not 

 until the thirtieth day that its first traces are manifest in the 

 existence of two currents, one running towards the head, the 

 other towards the trunk, (Fig. 122,) with similar returning 

 currents. At this time the liver begins to be formed. Mean- 

 while, the embryo gradually disengages itself, at both ends, 

 from its adherence to the yolk ; the tail 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 pul- 

 sating heart and circulating blood, and it moves its tail spon- 

 taneously. But the forms of the organs are not yet complete ; 

 nor have they yet acquired the precise shape that character- 

 izes the class, the family, the genus, and the species. The 

 young White-fish is as yet only a vertebrate animal in gen- 

 eral, and might as well 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 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 

 surrounds the body ; the anterior limbs, 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 



