1 88 NATURAL SCIENCE. Sept., 



the unwonted places bore the normal species-stamp as characteristi- 

 cally as similar parts grown in their normal places. It can hardly be 

 supposed that the architecture of the germ-plasm contains special 

 determinants to be ready for occurrences so casual, especially as 

 these are called into existence by circumstances quite foreign to the 

 normal environment of the organisms. On the other hand, the facts 

 are consonant with Hertwig's belief that, as all division is heirs-equal 

 division, all the species-characters that depend upon cells are latent 

 in every cell. 



The experiments of Driesch, Wilson, and Hertwig upon the early 

 stages of developing ova show that heteromorphosis begins with the very 

 earliest divisions of the egg. Thus Driesch, working upon echinoderm 

 embryos, was able to flatten out the stage where there was a sphere of 

 sixteen cells into a flat plate where all the cells were in the same 

 plane. In such a plate, the nuclei of the cells occupied relative 

 positions very different from the normal conditions. Yet from these 

 Driesch obtained normal plutei larvae. It was, in fact, as if the cells 

 could be pushed about like billiard balls without destroying the future 

 shape and characters of the embryo. Did each cell contain only the 

 determinants that would correspond to the structures that would arise 

 from it under its normal conditions, then change of its normal position 

 would have arrested development. Each cell must, on the other hand, 

 have contained the determinants for all the animal, and have allowed 

 those to come into operation that were required by the new positions 

 into which the cells were forced. Driesch, by separating the first two 

 and the first four segmentation-spheres of an Echinus ovum, obtained 

 two or four normal plutei, respectively one-half and a quarter of the 

 normal size. Here again each sphere must have contained all the 

 determinants for the whole organism. Heirs-equal division must 

 have occurred. So, also, in the case of Amphioxus, Wilson obtained a 

 normal, but proportionately diminished, embryo with complete nervous 

 system from a separated sphere of a two- or four- or eight-celled 

 stage. 



Hertwig himself, some years ago, published the results of experi- 

 ments he made upon the development of frogs' eggs under abnormal 

 conditions. He showed that there could be no question of imperative 

 divisions separating the germ-plasm into right and left halves, and 

 so forth, but that the method of division was determined by pressures 

 and relative gravities. Alteration of these made the ova divide into 

 novel but symmetrical forms. Chabry obtained normal embryos in 

 cases where some of the segmentation-spheres had been artificially 

 destroyed. 



These cases all show that in its possibilities each segmentation- 

 sphere is identical : that as a result of heirs-equal division, each 

 cell contains all the material necessary to cause the development of 

 a complete embryo. Weismann would have to suppose that in all 

 these cases, in addition to its half of the nuclear matter resulting 



