NATURE AND CAUSES OF DIFEERENTIATION 415 



ment. Hertwig insisted that the organism develops as a whole as 

 the result of a physiological interaction of equivalent blastomeres 

 the transformation of each being due not to an inherent specific 

 power of self-differentiation, as Roux's mosaic-theory assumed but 

 to the action upon it of the whole system of which it is a part 

 ''According to my conception," said Hertwig, - each of the first two 

 blastomeres contains the formative and differentiating forces not 

 simply for the production of a half-body, but for the entire organism ; 

 the left blastomere develops into the left half of the bodv only because 

 it is placed in relation to a right blastomere." 1 Again, in a later 

 paper : " The ^gg is a specifically organized elementary organism that 

 develops epigenetically by breaking up into cells and their subsequent 

 differentiation. Since every elementary part {i.e. cell) arises through 

 the division of the germ, or fertilized ^gg, it contains also the ger'm 

 of the w^hole, but during the process of development it becomes ever 

 more precisely differentiated and determined by the formation of 

 cytoplasmic products according to its position with reference to the 

 entire organism (blastula, gastrula, etc.)." 2 



An essentially similar view was advocated by the writer ('93, '94) 

 nearly at the same time, and the same general conception was ex- 

 pressed with great clearness and precision by Driesch shortlv after 

 Hertwig: "The fragments {i.e. cells) produced by cleavage are com- 

 pletely equivalent or indifferent." "The blastomeres of the sea- 

 urchin are to be regarded as forming a uniform material, and they 

 may be thrown about, like balls in a pile, without in the least degree 

 impairing thereby the normal power of development." '"^ '' The rehi- 

 tive positio7i of a blastomere iji tJie zi'/io/e determines in general what 

 develops from it ; if its position be e hanged, it gives rise to something 

 different; in other words, its prospective valne is a function of its 

 position. "^ 



Jn/this last aphorism the whole problem of development is brought 

 to a focus. It is clearly not a solution of the j^roblem, but (Mily a 

 highly suggestive restatement of it; for everything turns upon how 

 the relation of the part to the whole is conceived. W^-y little con- 

 sideration is required to show that this relation cannot be a merelv 

 geometrical or rudely mechanical one, for in the eggs of different 



1 '92, I, p. 481. 



^ '93» P- 793- It should be pointed out that Roux himself in several papers expressl\ 

 recognizes the fact that development cannot be regarded as a ]iure mosaic-work, and that 

 besides the power of self-differentiation postulated by his hypothesis we must assume a 

 "correlative differentiation" or differentiating interaction of parts in the embryo. Cf. Koux, 



'92, '93. I- 



3 Studien IV., p. 25. 



* Studien IV., p. 39. Cf. His, " Es muss die Wachsthumserregbarkeit dcs pjcs cine 

 Function des Raumes sein." ('74, p. 153.) 



