NATURE AND CAUSES OF DIFFERENTIATION 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 body only because 

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

 paper : " The egg 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 egg, it contains also the germ 

 of the whole, 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 shortly 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." 3 " The rela- 

 tive position of a blastomere in the whole determines in general what 

 develops from it ; if its position be changed^ it gives rise to something 

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

 position. " 4 



In this last aphorism the whole problem of development is brought 

 to a focus. It is clearly not a solution of the problem, but only a 

 highly suggestive restatement of it ; for everything turns upon how 

 the relation of the part to the whole is conceived. Very little con- 

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

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



1 '92, i, P . 4 8i. 



2 '93> P- 793- It should be pointed out that Roux himself in several papers expressly 

 recognizes the fact that development cannot be regarded as a pure 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. Roux, 



'92, '93, I- 



3 Studien IV., p. 25. 



4 Studien IV., p. 39. Cf. His, " Es muss die Wachsthumserregbarkeit des Eies eine 

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



