ON THE NATURE AND CAUSES OF DIFFERENTIATION 319 



Morgan, 1 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 cf 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 tlie axis of the 

 embryo is shifted. Oscar Schultze ('94) discovered that if the egg 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 whole half-sized dwarf might be formed, according to the 

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

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

 results, 2 precisely as described by Roux. If, on the other hand, the 

 blastomere be inverted, it may give rise either to a half-embryo 3 or to 

 a whole dwarf. 4 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 AmpJii- 

 oxus and the echinoderms. In Amphioxus 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 Toxopnenstes 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. Anz., X. 19, 1895. 3 Three cases. 



2 Eleven cases observed. 4 Nine cases observed. 



