NATURE AND CAUSES OE DIEFERENTIATION 423 



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 of both AvipJti- 

 oxns and the echinoderms. In Avipliioxus the isolated blastomere 

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

 however, not invariable, for a certain number of such blastomeres 

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

 of an entire embryo. The sea-urchin Toxopneustes reverses this rule, 

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

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

 it may segment like an entire ovum of half-size (Fig. 183, /^)and give 

 rise to an entire blastula. We may interpret this to mean that in 

 Avipliioxus the differentiation of the cytoplasmic substance is at first 

 very slight, or readily alterable, so that the isolated blastomere, as a 

 rule, reverts at once to the condition of the entire ovum. In the sea- 

 urchin, the initial differentiations are more extensive or more firmly 

 established, so that only exceptionally can they be altered. In the 

 snail and ctenophore we have the opposite extreme to Avipliioxus, the 

 cytoplasmic conditions having been so firmly established that they can- 

 not be readjusted, and the development must, from the outset, proceed 

 within the limits thus set up. 



Through this conclusion we reconcile, as I believe, the theories of \ 

 cytoplasmic localization and mosaic development with the hypothesis 

 of cytoplasmic totipotence. Primarily the egg-cytoplasm is totipotent y 

 in the sense that its various regions stand in no fixed relation with the \ 

 parts to which they respectively give rise, and the substance of each | 

 of the blastomeres into which it splits up contains all of the materialsj 

 necessary to the formation of a complete body. Secondarily, how- , 

 ever, development may assume more or less of a mosaic-like character ) 

 through differentiations of the cytoplasmic substance involving local/ 

 chemical and physical changes, deposits of metaplasmic material,/, 

 and doubtless many other unknown subtler processes. Both the ex-| 

 tent and the rate of such differentiations seem to vary in differentl 

 cases ; and here probably lies the explanation of the fact that the 

 isolated blastomeres of different eggs vary so widely in their mode 

 of development. When the initial differentiation is of small extent 

 or is of such a kind as to be readily modified, cleavage is indetervii- 

 nate in character and may easily be remodelled (as in Amphioxus). 

 When they are more extensive or more rigid, cleavage assumes a 

 mosaic-like or determinate character,^ and qualitative division, in a 

 certain sense, becomes a fact. Conklin's ('99) interesting observa- 

 tions on the highly determinate cleavage of g■A.s\.zxo^^o(i?. {Crepidiila) 



1 The convenient terms indeterminate and determinate cleavage were suggested by 

 Conklin ('98). 



