lO BIOLOGICAL LECTURES. 



we find, for example, among the annelids ; and I would ask 

 attention for a moment to the case of Nereis, which is, at 

 present, the best known form. Differentiation here begins at 

 the very first cleavage (which is conspicuously unequal), and 

 it becomes more pronounced with every succeeding division. 

 The median plane is marked out at the second cleavage ; at 

 the third the entire ectoblast of the trochal and prae-trochal 

 reo-ions is formed ; at the fourth the material for the entire 

 "ventral plate" (including the ventral nerve-cord and the seta- 

 sacs) is segregated in a single cell, that for the stomodaeum in 

 three cells ; the iifth cleavage completes the ectoblast, and by 

 the 38-celled stage the germ-layers are completely segregated 

 (the mesoblast in a single cell) and the architecture of the 

 embryo is fully outlined in the arrangement of the parent 

 blastomeres, or protoblasts. 



We do not know whether, in this case, the first two blasto- 

 meres are qualitatively different, though there may be some 

 ground for holding that they are, from the fact that the larger 

 of the two contains a relatively larger proportion of protoplasm 

 than the smaller.^ But in any case their difference in size 

 renders it impossible that they should play interchangeable 

 parts in the cleavage. The entire later development is, how- 

 ever, moulded upon the 2-celled stage, every blastomere having 

 a definite relation to it and a definite morphological value. The 

 development is a visible mosaic-work, not one ideally conceived 

 by a mental projection of the adult characteristics back upon 

 the cleavage stages. The principle of " organbildende Keim- 

 bezirke " has here a real meaning and value, and this would 

 remain true even if it should hereafter be shown that both of 

 the first two blastomeres of Nereis, if isolated, could produce a 

 l)erfect embryo. 



It is clear, from such a case, that the more extreme views 

 of Driesch and Hertwig cannot be accepted without consider- 

 able modification. It seems to me, however, that they may be 

 modified in such a way as, without sacrificing the principle of 

 epigenesis for which they contend, to recognize certain ele- 



1 All my attempts to separate these blastomeres by shaking have thus far been 

 unsuccessful. 



