Wolff illustrated the principle of gradual establishment 

 of parts in the process of development with many examples. 

 He referred to the description and drawings of Rezel, according 

 to which in the tadpoles of frogs^ there are no legs at the 

 beginning. The extremities are developed later. He said later 

 that, according to Reaumer, the accidentally destroyed chelae 

 of crayfish grow again and that Trembley saw simple polyps 

 which later became branched and complicated (his report, 

 apparently, is about the regeneration of antennae and about 

 the budding of hydra) . 



Describing the gradual formation of the intestinal canal 

 from the rolled-up and accreted edges of the membrane, Wolff 

 noticed, though obscurely, the similar formation of the 

 central nervous system. In fact, his analogy confirms the 

 general principle of gradual development for the commissure 

 of the false amnion, for the primary intestine, for the brain 

 and spinal cord, and even for the embryo as a whole. Wolff's 

 discovery of the common manner of development led to his 

 assumption of the transformation of that which is flat by 

 means of closure of its edges, into a hollow cylindrical 

 body. Apparently Wolff could not observe directly the process 

 of rolling up of the medullary membrane in a canal. In any 

 case, the propagation of the described principle of develop- 

 ment on the central nervous system, even if an assumption at 

 the time, was an excellent discovery which Baer made sixty 

 years later. 



In addition Wolff turned attention to the successive 

 formation of the different systems of organs, the first of 

 which, in his opinion, is the central nervous system. Then, 

 he supposed, the muscles are isolated. The third to become 

 clear is the circulatory system, and, finally, the digestive 

 system (p. 472) . 



From the third part of the reviewed work we must consider 

 only the general conclusions, which partly repeat those above, 

 but which deserve further mention because here Wolff brought 

 a final summation of the theory of epigenesis as opposed to 

 the idea of preexistence of the parts of the embryo. Thus, 

 he wrote : 



85 



