ON THE NA TURE AND CA USES OF DIFFERENTIA TION 3 1 3 



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, 1 but 

 during the process of development it becomes ever more precisely 

 differentiated and determined by the formation of cytoplasmic prod- 

 ucts according to its position with reference to the entire organism 

 (blastula, gastrula, etc)." 2 



Driesch expressed the same view with great clearness and pre- 

 cision shortly after Hertwig : " The fragments (i.e. cells) produced 

 by cleavage are completely equivalent or indifferent." "The blasto- 



Fig. 137. Diagrams contrasting the value of the blastomeres in polyclades and annelids. 



A. Plan of cleavage in the polyclade egg (constructed from the figures of Lang). B. Corre- 

 sponding plan of the annelid egg. In both cases the ectoblast is unshaded, with the exception of 

 X ; the mesoblast is ruled in vertical lines and the entoblast in horizontal. In both, three succes- 

 sive quartets of micromeres are budded forth from the four primary cells A. B. C. D. In the 

 polyclade the first quartet is ectoblastic, the second and third mesoblastic. In the annelid all three 

 quartets are ectoblastic, while the mesoblast (M) arises from the posterior cell of a fourth quartet 

 of which the remaining three are entoblastic. 



meres 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 develop- 

 ment." 3 "The relative position of a blastomere in the whole de- 

 termines in general wJiat develops from it ; if its position be cJianged, 

 it gives rise to something different ; in other words, its prospective 

 value is a function of its position." 4 



This conclusion undoubtedly expresses a part of the truth, though, 

 as will presently appear, it is too extreme. The relation of the part 



1 That is, in the specifically organized chromatin within the nucleus. 



2 '93 P- 793- 3 Studien IV. p. 25. 4 Studien TV. p. 39. 



