20 THE ANATOMY OF INVERTEBRATED ANIMALS. 



Caspar F. Wolff furnished further, and indeed conclusive, 

 proof of the truth of the theory of epigenesis ; but, unfortu- 

 nately, the authority of Haller and the speculations of Bonnet 

 led science astray, and it was reserved for Von Baer to put the 

 nature of the process of development in its true light, and to 

 formulate it in his famous law. 



3. Development, then, is a process of differentiation by 

 which the primitively similar parts of the living body become 

 more and more unlike one another. 



This process of differentiation may be effected in several 

 ways: 



(1.) The protoplasm of the germ may not undergo divi- 

 sion and conversion into a cell aggregate ; but various parts 

 of its outer and inner substance may be metamorphosed di- 

 rectly into those physically and chemically different materials 

 which constitute the body of the adult. This occurs in such 

 animals as the Infusoria, and in such plants as the unicellular 

 Algce and Fungi. 



(2.) The germ may undergo division, and be converted 

 into an aggregate of division masses, or blastomeres, which 

 become cells, and give rise to the tissues by undergoing a 

 metamorphosis of the same kind as that to which the whole 

 body is subjected in the preceding case. 



The body, formed in either of these ways, may, as a whole, 

 undergo metamorphosis by differentiation of its parts ; and 

 this differentiation may take place without reference to any 

 axis of symmetry, or it may have reference to such an axis. 

 In the latter case, the parts of the body which become dis- 

 tinguishable may correspond on the two sides of the axis (bi- 

 lateral symmetry), or may correspond along several lines paral- 

 lel with the axis (radial symmetry). 



The bilateral or radial symmetry of the body may be fur- 

 ther complicated by its segmentation, or separation by divi- 

 sions transverse to the axis, into parts, each of which corre- 

 sponds with its predecessor or successor in the series. 



In the segmented body, the segments may or may not give 

 rise to symmetrically or asymmetrically disposed processes, 

 which are appendages, using that word in its most general 

 sense. 



And the highest degree of complication of structure, in 

 both animals and plants, is attained by the body when it be- 

 comes divided into segments provided with appendages ; when 

 the segments not only become very different from one another, 

 but some coalesce and lose their primitive distinctness ; and 



