26 president's address. 



pile, without the least impairment of their power of development." 

 " Their prospective value," according to Driesch, "is a function, 

 of their position in the whole," which, in this connection, means 

 their morphological relations to each other. Yet it has been 

 shown in a large number of cases at a very early stage of 

 development, and in some cases even from the first, there exists 

 a degree of qualitative diflferentiation of the germ-material. 

 Such was shown to be present in Amphioxus and Nereis by E. B. 

 Wilson, several years ago. And if the fact of the regeneration 

 of the missing halves of hemi-embryos proved fatal to the mosaic 

 theory of development in its original form, more recent observa- 

 tions have shown that a fairly extensive predetermination of 

 cytoplasmic regions may in certain cases be shown to exist. 

 Thus in the case of the egg of Beroe, the experiments of Driesch 

 and Morgan, and more lately those of Fischel, have shown that 

 an isolated blastomere of the two- or four-celled stage gives rise to 

 a half- or quarter-embryo; and also if part of an unsegmented egg 

 were removed the rest generated an incomplete larva, showing 

 certain defects which represent the portions removed. 



A conclusive evidence of underlying mechanical arrangements 

 in germinal structure would seem, moreo^'er, to be derivable from 

 experiments upon the influence of gravity upon the development of 

 frogs' ova. In 1894, O. Schultze discovered that if the egg of a 

 frog be turned upside down when in the two-cell stage, a whole 

 embryo, (or half of a double embryo) might arise from each blasto- 

 mere instead of a half-embryo, as in the normal development, and 

 that the axes of these embryos show no constant relation to one 

 another. Again, if, after destruction of one blastomere, the 

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

 always results, precisel}' 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. According to Wilson, from whom 

 I have largely quoted in reference to these experiments, we have 

 here the most conclusive evidence that each of the two blasto- 

 meres contains all the materials, nuclear and cytoplasmic, neces- 

 sary for the formation of a whole body; and that these materials 



