56 THE EIDDLE OF THE UNIVERSE. 



In the month of November, 1759, a young doctor of 

 twenty-six years, Caspar Friedrich Wolff (son of a 

 Berlin tailor), published his dissertation for the degree 

 at Halle, under the title Theoria Generationis. Sup- 

 ported by a series of most laborious and painstaking 

 observations, he proved the entire falsity of the 

 dominant theories of preformation and scatulation. 

 In the hatched egg there is at first no trace of the 

 coming chick and its organs ; instead of it we find on 

 top of the yolk a small, circular, white disk. This 

 thin " germinal-disk " becomes gradually round, and 

 then breaks up into four folds, lying upon each other ; 

 these are the rudiments of the four chief systems of 

 organs — the nervous system above, the muscular 

 system underneath, the vascular system (with the 

 heart), and, finally, the alimentary canal. Thus, as 

 Wolff justly remarked, the embryonic development 

 does not consist in an unfolding of pre-formed organs, 

 but in a series of new constructions ; it is a true 

 epigenesis. One part arises after another, and all 

 make their appearance in a simple form, which is 

 very different from the later structure. This only 

 appears after a series of most remarkable forma- 

 tions. Although this great discovery — one of the 

 most important of the eighteenth century — could be 

 directly proved by a verification of the facts Wolff 

 had observed, and although the " theory of genera- 

 tion " which was founded on it was in reality not a 

 theory at all, but a simple fact, it met with no 

 sympathy whatever for half a century. It was 

 particularly retarded by the high authority of Haller, 

 who fought it strenuously with the dogmatic assertion 

 that ' ' there is no such thing as development : no 

 part of the animal body is formed before another ; all 



