186 VITALISM AND SCHOLASTICISM 



These were made on that favourite subject 

 of research, the developing ovum of the sea- 

 urchin. This forms a blastula of about one 

 thousand cells a hollow sphere as we have 

 seen. If, with a pair of very fine scissors, this 

 blastula is cut up into bits, in any direction 

 but so that no fragment is less in size than one 

 quarter of the entire blastula, each such bit will 

 go on developing and will form a complete, 

 though small, larva. Now let us turn to the 

 bearing of these experiments. Driesch very 

 D i h properly says that one proof is sufficient. " A 

 truth is either proved or not proved; and, if 

 it is once proved, it is not necessary to prove 

 it further," * and the first and most important 

 of his three proofs is based on the experiments 

 which we have been considering. This must 

 now be detailed in a paraphrase of his own 

 words, t Fragments of the blastula, as well 

 as the first two or four cleavage cells can give 

 complete embryos even though the former 

 fragments are cut quite at random. The pro- 

 spective potency then not only of the first two 

 or four cleavage cells but also of all the 

 thousand cells of the blastula must be the same. 

 To a collection of cells each of which has an 

 equal prospective potency he gives the name 



* Problem of Individuality, p. 20. 

 t Chiefly from the work just quoted, pp. 13 seq. 



