41 6 INHERITANCE AND DEVELOPMENT 



animals blastomeres may almost exactly correspond in origin and 

 relative position, yet differ widely in their relation to the resulting 

 embryo. Thus we find that the cleavage of polyclades, annelids, and 

 gasteropods (Fig. i88) shows a really wonderful agreement in form, 

 yet the individual cells differ markedly in prospective value. In all 

 of these forms three quartets of micromeres are successively formed 

 according to exactly the same remarkable law of the alternation of the 

 spirals ; ^ and, in all, the posterior cell of a fourth quartet lies at the 

 hinder end of the embryo in precisely the same geometrical relation 

 to the remainder of the embryo ; yet in the gasteropods and annelids 

 this cell gives rise to the mesoblast-bands and their products, in the 

 polyclade to a part of the archenteron, while important differences 

 also exist in the value of the other quartets. The relation of the 

 part to the whole is therefore of a highly subtle character, the pro- 

 spective value of a blastomere depending not merely upon its geomet- 

 rical position, but upon its relation to the whole complex inherited 

 organization of which it forms a part. The apparently simple con- 

 clusion stated in Driesch's clever aphorism thus leads to further prob- 

 lems of the highest complexity. It should be here pointed out that 

 Driesch does not accept Hertwig's theory of the interaction of blasto- 

 meres as such, but, hke Whitman, Morgan, and others, has brought 

 forward effective arguments against that too simple and mechanical 

 conception. That theory is, in fact, merely Schwann's cell-composite 

 theory of the organism applied to the developing embryo, and the 

 general arguments against that theory find some of their strongest 

 \ support in the facts of growth and development.^ Th]s has been 

 ' forcibly urged by Whitman ('93), who almost simultaneously with the 

 statements of Driesch and Hertwig, cited above, expressed the con- 

 viction that the morphogenic process cannot be conceived as merely 

 the sum total or resultant of the individual cell-activities, but operates 

 as a unit without respect to cell-boundaries, precisely as De Bary con- 

 cludes in the case of growing plant-tissues (p. 393), and the nature 

 of that process is due to the organization of the ^gg as a whole. ^ 



While recognizing fully the great value of the results attained 

 during the past few years in the field of experimental and specula- 

 tive embryology, we are constrained to admit that as far as the 

 essence of the problem is concerned we have not gone very far 

 beyond the conclusions stated above ; for beyond the fact that the 

 inherited organization is involved in that of the germ-cells we remain 

 quite ignorant of its essential nature. This has been recognized by 

 no one more clearly than by Driesch himself, to whose critical 

 researches we owe so much in this field. At the cUmax of a recent 

 elaborate analysis, the high interest of which is somewhat obscured by 

 1 Cf. p. 368. ^ Cf. pp. 388-394. 



