3o6 OF THE GROWTH, &c. 



vertebrae, as the fmall branches fhoot out from 

 the principal branch of a tree. In all embryos, 

 the middle of the head and the vertebrae firll 

 appear; w^e then fee, on each fide of the veficle 

 which compofes the middle of the head, two o- 

 ther veficles which feem to proceed from the 

 firfl : Thefe two veficles contain the eyes, and 

 the other double parts of the bead. In the fame 

 manner, we perceive an equal number of fmall 

 tubercles iffuing from each fide of the vertebrae, 

 which gradually extend, and form the ribs, and 

 other double parts of the trunk. Laflly, the 

 arms and legs appear like fmall tubercles on 

 each fide of the trunk. Tliis firft growth ia 

 verv different from what afterwards takes place ; 

 It is the produdion of parts which appear for 

 the firft time; the growth which fucceeds is on- 

 ly an expanfion of parts already formed. 



The order and fymmetry of the double parts 

 in all animals, their regular pofition, the equa- 

 lity of their extenfion and growth, and the per- 

 fect fimilarity of their ftrudure, feem to indi- 

 cate, that they derive their origin from the 

 fingle parts ; that a certain force refides in the 

 fmgle parts which ads equally on each fide ; or, 

 which amounts to the fame, they are the bafes or 

 fulcra againft which the adion of thofe powers 

 that produce the expanfion of the double parts is 

 exerted; and that the adion of thefe forces, 

 t>oth on the right and left fides, are precifely 

 equal, and counterbalance each other. 



Hence 



