NATURE AND CAUSES OE DIFEERENTIATION 417 



its too abstruse form, Driesch can only reiterate his former aphorism,^ 

 finally taking refuge in an avowed theory of vitalism which assumes 

 the localization of morphogenic phenomena to be determined bv "a 

 wholly unknown principle of correlation,"^ and forms a problem sni 

 generis^^ This conclusion recognizes the fact that the fundamental 

 problem of development remains wholly unsolved, thus confirming 

 from a new point of view a conclusion which it is only fair to point 

 out has been reached by many others. 



- But while the fundamental nature of the morphogenic process thus 

 remains unknown, we have learned some very interesting facts regard- 

 ing the conditions under which it ta'kes place, and which show that 

 Driesch's aphorism loses its meaning unless carefully qualified. The 

 experiments referred to at pages 353, 410, show that up to a certain 

 stage of development the blastomeres of the early echinoderm, AnipJii- 

 oxiis or medusa-embryo, are "totipotent" (Roux), or "equipotential " 

 (Driesch), i.e. capable of producing any or all parts of the body. 

 Even in these cases, however, we cannot accept the early conclusion 

 of Pfliiger i^'^Z), applied by him to the frog's ^gg, and afterward 

 accepted by Hertwig, that the material of the ^gg,, or of the blasto- 

 meres into which it splits up, is absolutely " isotropic," i.e. consists of 

 quite uniform indifferent material, devoid of preestablished axes. 

 Whitman and Morgan, and Driesch himself, showed that this cannot 

 be the case in the echinoderm Qgg\ for the ovum possesses a polarity 

 predetermined before cleavage begins, as proved by the fact that at 

 the fourth cleavage a group of small cells or micromeres always arises 

 at a certain point, which may be precisely located before cleavage by 

 reference to the eccentricity of the first cleavage-nucleus,'* and which, 

 as Morgan showed,^ is indicated before the third, and sometimes 

 before the second cleavage, by a migration of pigment away from the 

 micromere-pole. These observers are thus led to the assumption of 

 a primary polarity of the egg-protoplasm, to which Driesch, in the 

 course of further analysis of the phenomena, is compelled to add the 

 assumption of a secondary polarity at right angles to the first.'^ The.se 

 polarities, inherent not only in the entire ^gg, but also in each of the 

 blastomeres into which it divides, form the primary conditions under 

 which the bilaterally symmetrical organism develops by epigenesis. 

 To this extent, therefore, the material of the blastomeres, though 

 ''totipotent," shows a certain predetermination with respect to the 

 adult body. 



1 '99, pp. 86-87. 



2 This phrase is cited by Driesch from an earher work (^'92, p. 59O) as ^ivm- a cncct 

 though " unanalytical " statement of his view. It may be questioned whether many readers 

 will regard as an improvement the '' analytical " form it assumes in his last work. 



3 I.e., p. 90. * 'Cf. Fig. 103. ^ '94, I'. 142. 

 6 See Driesch, '93, pp. 229, 241 ; '96, and '99, p. 44- 



2 E 



