116 



EMBRYOLOGY. 



depression is formed in it (Fig. 114). This depression 

 becomes by degrees a deep furrow, and soon after a second 

 furrow appears at right angles with the former, so that the 

 germ now presents four elevations (Fig. 115). The subdi- 

 vision goes on in this way, during the second and third 

 days, until the germ is divided into numerous little spheres, 

 giving it the appearance of a mulberry (Fig. 116). 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. 



309. 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 are constantly ap- 

 proaching each other until they unite and form a canal (Fig. 

 117, ft), as has been before shown. At the same time 



Fig. 117. 



Ficr. 118. 



Fig. 119. 



an enlargement of one of the extremities 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 (/?i), hearing (e), and smell (jo). 



310. Towards the thirteenth day we see, in the place af- 

 terwards occupied by the back-bone, a transparent, cartilag- 

 inous cord, composed of large cells, on which transverse 

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

 is the dorsal cord, an organ which, as we have before seen, 



