THE MECHANISM OF LIFE 535 



Roux's own object, the frog's egg. That definite parts or 

 organs of the embryo do not develop from single cleavage-cells 

 arising by the division of the ovum, Driesch and Hertwig were able 

 to show by continuing in a different way the experiment performed 

 by Pfliiger. They, like Pfluger, clamped frogs' eggs between two 

 glass plates in such a way that the cells arising in division were 

 able to arrange themselves in one plane only instead of in a 

 spherical mass, so that they were abnormally placed with 

 reference to one another. Notwithstanding this, completely normal 

 embryos developed from them. From this fact the conclusion must 

 necessarily be drawn that the individual cells arising in cleavage 

 do not represent definite rudiments of organs, and that no organ- 

 forming germ-regions can be present in the ovum. 



To summarise briefly the contrasts that are expressed in these two 

 wholly opposed theories, the idea of Weismann and Roux is essen- 

 tially the old, more or less clearly expressed doctrine of preformation 

 of the time of Haller, in somewhat modern garb, while the view of 

 Pfluger, Hertwig, and Driesch represents the standpoint of the 

 doctrine of epigenesis of Caspar Friedrich Wolff, which Haeckel, 

 more than all others, has constantly maintained with great 

 persistence in the later embryology. As thus contrasted, the two 

 doctrines are incompatible with one another. There can be no 

 doubt that the facts are adverse to such a very minute preformation 

 of organ-forming germ -regions in the egg, as especially Weismann 

 and De Vries ('89 ) have assumed. The two facts, first, that small 

 pieces of an egg-cell and isolated halves and quarters, formed in 

 cleavage, produce a normal, complete organism of a correspondingly 

 small size; and, secondly, that, when the cleavage-cells are 

 displaced, animals develop with their organs in a completely normal 

 position, these facts prove that the different parts of the egg-cell 

 must be of absolutely equal value in the production of the cells, tissues 

 and organs proceeding from them, and that we are not justified in 

 speaking of a localized preformation of definite rudiments in the 

 egg ; it makes no difference whether we assume 10, 100, or 1000 

 rudiments, as Roux does, or several billions. 1 While, further, the 



1 Since Roux has protested against being reckoned among the preformationists, 

 where he has been placed not only by myself, but by many (I think most) investi- 

 gators belonging to his special field, in order to do him no injustice I ought 

 not to leave his protest unmentioned. But, at the same time, I must say that 

 upon the basis of his own work I have not been able to alter my foregoing judgment 

 and to accept his explanation. Since, as Roux himself acknowledges, " there are 

 at present few authors who know my [his] views clearly," in order to give the 

 reader the opportunity of an independent judgment concerning them, I will quote 

 his own words, in which his standpoint is stated. In Virchow's Archiv, vol. 

 cxiv, 1888, as well as in the Verhandlungen der anntomischen Gesellschaft auf der 

 sechsten Versamrrdung in Wien, 1892, Roux summarises the results of his experi- 

 ments and speculations, and explains that "cleavage divides quantitatively the 

 part of the germinal material that accomplishes the direct development of the in- 

 dividual, especially the nuclear material, and, by means of the arrangement thus 

 made of the various separate materials, determines at once the position of the later 



