146 



EMBRYOLOGY. 



globules, a swelling in the shape of a transparent vesicle, 

 (Fig. 113, g,) composed of very delicate cells. This is the 

 first indication of the germ. This swelling rapidly enlarges 

 until it envelops a great part of the yolk, when a depression 



Fig. 114. 



Fig. 115. 



Fig. 116. 



is formed upon it, (Fig. 114.) This depression becomes by 

 degrees a deep furrow, and soon after a second furrow ap- 

 pears at right angles with the former, so that the germ now 

 presents four elevations, (Fig. 115.) The subdivision goes 

 on in this way, during the second and third days, until the 

 germ is divided into numerous little spheres, giving the sur- 

 face the appearance of a mulberry, (Fig. 116.) This ap- 

 pearance, 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 constantly approach 



Fig. 117. Fig. 118. Fig. 119. 



each other until they unite and form a canal, (Fig. 117, Z>, 



