DEVELOPMENT OF THE YOUNG WITHIN THE EGG. L'Sf) 



which is the rudiment of the ear. At this period, the dis- 

 tinction between the upper and the lower layer of the germ is 

 best traced ; all the changes mentioned above appertaining to 

 the upper layer. 



467. After the seventeenth day, the lower or mucous layer 

 divides into two sheets, the inferior of which becomes the in- 

 testine ; the heart shows itself about the same time, under the 

 form of a simple cavity (fig. 307, /?), 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 observed, and the globules of blood are 

 seen to rise and fall in conformity with these motions. 



Fig. 306. 



Fig 307. 



Ficr. 308. 



468. 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. 308), with similar returning cur- 

 rents. At this time the liver begins to form. Meanwhile the 

 embryo gradually disengages itself at both extremities from its 

 adherence to the yolk ; the tail becomes free, and the young 

 animal moves it in violent jerks. 



469. The embryo, although still inclosed in the egg, now 

 unites all the essential conditions for the exercise of the func- 

 tions 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 

 tliev acquired the precise shape characterizing the class, the 

 family, the genus, and the species. The young white-fish is as 

 yet only a vertebrate animal in general, and might be taken 

 for the embryo of a frog. 



