THE VITAL IMPETUS 139 



same extent, and at the same rate, and corresponding 

 parts may fail to come into contact with each other. 

 Lacking, then, the normal stimulus of the other part, 

 each blastomere begins to develop by itself, and a 

 double embryo is produced. It is clear, then, both 

 from this case and the last one, that the actual fate 

 of any one part of the system of blastomeres is a 

 function of its position. What it will become depends 

 precisely on where it is situated with respect to the 

 other parts. 



Driesch, then, calls the system of parts in such cases 

 as the 2 -cell frog embryo, or the i6-cell sea-urchin 

 embryo, an equipotential system, since each part is 

 potentially able to do what any other part may do, 

 and what the whole system may do. But in normal 

 development each part has a definite fate and its 

 activity is co-ordinated with that of all the other parts. 

 It is, therefore, an harmonious equipotential system, each 

 part acting in harmony, and towards a definite result, 

 with all the others ; although if necessary it can take 

 the place of any or all of the others. 



Such an harmonious equipotential system exists 

 only at the beginning of the development of the egg. 

 It is represented by the 8-cell stage of Echinus but 

 not by the i6-cell stage, since, though the rir-blasto- 

 meres produce gastrulae (the first larval stage) , they do 

 not produce plutei (the second stage). It is repre- 

 sented by the 4-cell stage of Amphioxus but not by 

 the 8-cell stage. It is not exhibited even by the 

 2-cell stage of the Ctenophore egg. What does this 

 mean ? It means that the further development pro- 

 ceeds, the less complete does the " organisation " 

 inherent in any one part of the system become. " The 

 ontogeny assumes more and more the character of a 

 mosaic work as it proceeds ' (Wilson) . 



