78 THE FIRST PRINCIPLES OF HEREDITY 



tatively unequal (erb-ungleich) division of the germ-plasm 

 takes place during development, by which a different set 

 of determinants is allotted to each cell, thus deciding time 

 and mode of its development, though such differences may 

 not be visible with our means of observation. Only when 

 a cell divides into a mass of cells, all of the same function, 

 does an integral or doubling (erb-gleich) division of the 

 nuclear substance take place. Hertwig, on the other hand, 

 denies the occurrence of a differentiating division altogether 

 on many grounds, which it is impossible to enumerate here 

 in detail. He holds that all cells are of equal value, it only 

 depending on their relative position to each other what part 

 of the organism arises from them. Unfortunately, experi- 

 ments conducted with a view of settling this question have 

 not been decisive either way. If the hereditary substance 

 of the ovum has a definite architecture, and each cell, as it 

 appears, receives its predetermined part of it, then, if any 

 of the cleavage-cells are removed, the corresponding part 

 of the embryo would be removed with it, and a defective 

 embryo be the result. Indeed, Roux's experiments on 

 frogs' eggs showed that the removal of one of the first two 

 cleavage-cells leads to a one-sided embryo. But, on the 

 other hand, experiments by Hertwig, Driesch, and others 

 have given the opposite result, blast omeres (separated 

 cleavage-cells) of the first stages developing into normal 

 though diminutive embryos. Not only this, but artificial 

 influences, as pressure on the developing ovum, may bring 

 about a change in the order of its cell-segmentation, so 

 that now differently situated cells form tissues previously 

 formed by other cells. In short, Hertwig considers that 

 " The motor forces of development are not residing in the 

 germ-plasm, but external to it ; they are due to the con-, 

 tinual changes in the mutual relations of the cells as they 

 increase by division, and to influences of the surroundings." 

 It is quite possible, though, that in different cases the final 

 differentiation of cells may not occur at the same period of 

 segmentation, so that in the first stages the blastomeres 



