DEVELOPMENT OF THE YOUNG WITHIN THE EGG. 147 



as has been before shown, (Fig. 107.) At the same time, 

 an enlargement at one end of the furrow is observed. This 

 is the rudiment of the head, (Fig. 118,) in which may soon 

 be distinguished traces of the three divisions of the brain, 

 (Fig. 119,) corresponding to the senses of sight, (m,) hear- 

 ing, (e,) and smell, (p.) 



310. Towards the thirteenth day, we see a transparent, 

 cartilaginous cord, in the place afterwards occupied by the 

 back-bone, composed of large cells, on which transverse 



Fig. 120. 



Fig. 121. 



Fig. 122. 



divisions are successively forming, (Figs. 120, 121, e.) This 

 is the dorsal cord, a part of which, as we have before seen, 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 embryonic state 

 is permanent through life, and no true back-bone is ever 

 formed. Soon after, the first rudiments of the eye appear, 

 in the form of a fold in the external membrane of the germ, 

 in which the crystalline lens (Fig. 121, x) is afterwards 

 formed. At the same time we see, at the posterior part 

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

 ear. At this period, the distinction between the upper and 

 the lower layer of the germ is best traced ; all the changes 

 mentioned above appertaining to the upper layer. 



311. After the seventeenth day, the lower layer divides 

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



