DEVELOPMENT OF THE YOUNG WITHIN THE EGG. 117 



is common to all embryos of vertebrated animals. It 

 always precedes the formation of the back-bone ; and in 

 some fishes, as the sturgeon, this cartilaginous or embry- 

 onic state is permanent through life, and no true back-bone 

 is ever formed. Soon after, the first rudiments of the eye 

 appear, being a fold in the external membrane of the 

 germ, in which the crystalline lens (Fig. 121, #) is after- 

 wards formed. At the same time we see at the posterior 

 part of the head an elliptical vesicle, which is the rudiment 

 of the ear. 



311. After the seventeenth day, the mucous layer divides 

 into two sheets, the inferior of which becomes the intestine. 

 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 glo- 

 bules 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 sim- 



-k, 



Fisr. 120. 



Fisr. 121. 



Fig. 122. 



ilar returning currents. 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 



