ON THE NATURE AND CAUSES OF DIFFERENTIATION 319 



Morgan,^ was reached in the following manner. Born had shown, in 

 1885, that if frogs' eggs be fastened in an abnormal position, — e.g. 

 upside down, or on the side, — a rearrangement of the egg-material 

 takes place, the heavier deutoplasm sinking towards the lower side, 

 while the nucleus and protoplasm rise. A new axis is thus established 

 in the egg, which has the same relation to the body-axes as in the 

 ordinary development (though the pigment retains its original arrange- 

 ment). This proves that in eggs of this character (telolecithal) the 

 distribution of deutoplasm, or conversely of protoplasm, is one of the 

 primary formative conditions of the cytoplasm; and the significant 

 fact is that by artificially changing this distribution the axis of the 

 embryo is shifted. Oscar Schultze ('94) discovered that if the ^^^ be 

 turned upside down when in the two-cell stage, a whole embryo (or 

 half of a double embryo) might arise from each blastomere instead 

 of a half-embryo as in the normal development, and that the axes of 

 these embryos show no constant relation to one another (Fig. 140). 

 Morgan ('95,3) added the important discovery that either a half- 

 embryo or a w^hole half-sized dwarf might be formed, according to the 

 position of the blastouiere. If, after destruction of one blastomere, the 

 other be allowed to remain in its normal position, a half-embryo always 

 results,^ precisely as described by Roux. If, on the other hand, the 

 blastomere be inverted, it may give rise either to a half-embryo ^ or to 

 a whole dwarf.^ Morgan therefore concluded that the production of 

 whole embryos by the inverted blastomeres was, in part at least, due 

 to a rearrangement or rotation of the egg-materials under the influence 

 of gravity, the blastomere thus returning, as it were, to a state of 

 equilibrium like that of an entire ovum. 



This beautiful experiment gives most conclusive evidence that each 

 of the two blastomeres contains all the materials, nuclear and cyto- 

 plasmic, necessary for the formation of a whole body ; and that these 

 materials may be used to build a whole body or half-body, according 

 to the grouping that they assume. After the first cleavage takes 

 place, each blastomere is set, as it were, for a half-development, but 

 not so firmly that a rearrangement is excluded. 



I have reached a nearly related result in the case both of Aniphi- 

 oxHS and the echinoderms. In Aniphioxns the isolated blastomere 

 usually segments like an entire ovum of diminished size. This is, 

 however, not invariable, for a certain proportion of the blastomeres 

 show a more or less marked tendency to divide as if still forming part 

 of an entire embryo. The sea-urchin Toxopncustes reverses this 

 rule, for the isolated blastomere of the two-cell stage usually shows a 

 perfectly typical half-cleavage, as described by Driesch, but in rare 



1 Anat. Aiiz., X. 19, 1895. ^ Three cases. 



'^ Eleven cases observed. * Nine cases observed. 



