284 EMBRYOLOGY. 



four elevations (fig. 301). The subdivision goes on in this 

 way during the second and third days, until the germ is 

 divided into numerous little spheres, giving the surface the ap- 

 pearance of a mulberry (fig. 302). This appearance, however, 

 does not long continue ; at the end of the third day, the fissures 

 again disappear, and leave no visible traces. After this, the 

 germ continues to extend as an envelop around the yolk, which 

 it at last entirely encloses. 



465*. On the tenth day, the first outlines of the embryo 

 begin to appear, and we soon distinguish in it a depression 

 between two little ridges, whose edges constantly approach 

 each other until they unite and form a canal (fig. 303, b), as 

 has been before shown (fig. 293). At the same time an en- 

 largement at one end of the furrow is observed. This is the 

 rudiment of the head (fig. 304), in which may soon be dis- 

 tinguished traces of the three divisions of the brain (fig. 305), 

 corresponding to the senses of sight (m), hearing (<?), and 

 smell (p)> 



Fig. 303. Fig. 304. Fig. 305. 



466. Towards the thirteenth day we see a transparent, 

 cartilaginous cord, in the place afterwards occupied by the 

 back-bone, composed of large cells, in which transverse di- 

 visions are successively forming (figs. 306, 307, c). This is 

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

 common to all embryos of the vertebrated animals. It always 

 precedes the formation of the back-bone ; and in some fishes, 

 as the sturgeon (fig. 374), 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. 307, x) is afterwards formed. At the same 

 time we see at the posterior part of the head an elliptical vesicle, 



