CHAP, iv.] 



OF ANIMAL GENERATION. 



329 



A very important general fact, of which the partisans of the 

 evolutive or Darwinian doctrines make great use, and with y 

 justice, is the identity of the embryonary forms at the outset J 

 in all the vertebrates, and the appearance in a regular and/ 



FIG. 51. 



Ovum of dog. The embryoa in form of 



shoe sole is outlined. 

 o, dorsal fissure ; 

 fc, dorsal plates ; 



c, clear area ; * 



d, opaque germinative area ; 



e, membrane of the germinative vesicle. 

 The small superior figure is of natural size. 



The inferior figure is magnified. 



FIG. 52. 



Embryon of a dog's ovum, twenty days old. 

 Dorsal fissure widely open and surrounded 

 everywhere with a bright edge, which indi- 

 cates the first deposit of the nervous sub- 

 stance on the depth and the walls of the 

 fissure. In the depth is seen on the median 

 line the chorda dorsalis represented by a 

 darker stripe. a, ft, c, rudiments of the cere- 

 bral vesicles ; e, posterior rhomboidal sinus ; 

 d, body of primordial vertebrae ; /, lateral 

 plates ; g, middle and external vestments of 

 the blastodermic vesicle, still united; h, 

 mucous vestment; i, body of the embryon. 



successive order of the characteristics of each type. We may 

 compare the embryons of the vertebrates to a group of travellers 

 starting from the same place, and entering first of all a vast 

 general route, which they forsake to enter roads of more and more 

 secondary importance, and diverging less and less. It is first of 



