580 THE HISTORY OF BIOLOGY 



and whether this predestination can be affected by different influences. 

 This problem, in fact, is the old antithesis of preformation and epigenesis 

 formulated in a different way, and the discussion of the problem has to a 

 great extent been carried on in the new form brought about by the employ- 

 ment of the methods of experimental embryology. Roux himself, starting 

 from the theory of the continuity of the germinal plasm, which was in- 

 fluenced by Weismann and Sachs, maintained the idea of a very early deter- 

 mination of the various parts of the embryo; in his view, the first cleavage 

 plane of the germinal egg establishes the median plane in the individual, 

 the cleavage furrow is determined by the line of penetration of the sperm, 

 and the front part of the future animal is already determined before the fer- 

 tilization of the egg through the amassing of cytoplasm in that quarter. 

 Each successive cleavage delimits a prospective part of the embryo; be- 

 cause in karyokinesis the substance becomes not equally apportioned to the 

 daughter-nuclei, these latter acquire different values, so that, as he says, the 

 whole process of development becomes a piece of mosaic work. In proof of 

 his theory Roux carried out a series of experiments, which excited universal 

 admiration at the time; he treated a newly-fertilized frog's egg in such a 

 way that with a heated needle he burnt away one of the two first-formed 

 blastomeres; then there developed at first a half-embryo, which afterwards re- 

 generated in the usual manner of the Amphibia. Roux performed many other 

 experiments with the same purpose in view, and though his technique was 

 simple as compared with that which his successors afterwards elaborated, 

 it was nevertheless he who first systematized this kind of research work. 

 Roux, however, was at once subjected to sharp criticism; O. Hertwig 

 in particular, who from the very beginning had been an opponent of 

 the Weismann heredity-theory and its founders, at once attacked the 

 "mosaic" theory, maintaining that the different parts of the egg are 

 by no means predetermined, but that, on the contrary, the egg's mass 

 is equipotential-isotopic, as he calls it. Moreover, Hertwig, who in- 

 variably opposed narrow mechanistic tendencies in biology, strongly ob- 

 jected to the actual term "developmental mechanics," which appeared to 

 him to imply an inadmissible schematizing of the phenomena of life. And, 

 finally, he considered Roux's experiments to be inaccurate; he imitated them 

 and found that the halved egg gave rise, not to a half-embryo, but to a 

 whole embryo of small size. Hertwig found immediate support in Driesch, 

 an observer who will be mentioned further in another connexion. He halved 

 newly-segmented eggs of the sea-urchin and obtained from each half one 

 larva half the size of the normal. On the basis of these and other experiments 

 he propounded a special theory of evolution, according to which each part 

 of the egg has, on the one hand, a "prospective value," and, on the other 

 hand, a "prospective potentiality"; the meaning of these terms will best 



