DIFFERENTIAL DIVISION 423 



marvel of it, until we play some trick with the developing ^^g^ 

 introducing disorder, and see how equilibrium and normality 

 are restored. Thus the one-sided half-embryo of the frog 

 proceeds at a certain stage to develop the missing half. 



Roux. — Starting from the assumption that the nuclei of the 

 germ-ceUs contain a specific idioplasm or architectural sub- 

 stance (the vehicle of the heritable qualities), and with the 

 further assumption that this substance is a complex aggregate 

 of different kinds of particles (the material expressions of 

 different sets of qualities), Roux invented the hypothesis of two 

 kinds of nuclear division, quantitative and qualitative,. The 

 former results in equivalent, the latter in dissimilar nuclei ; the 

 former is an integral, the latter a differential division. Roux 

 supposed that the latter mode was characteristic of the early 

 stage of development, during which the different components 

 or quahties of the idioplasm are distributed among the blas- 

 tomeres. Thus it comes about that each blastomere, though 

 not independent of its neighbours, is endowed with a power 

 of " self-differentiation " along particular lines defined by its 

 specific share of the idioplasm. 



Weismann. — Similarly, but even more elaborately, as we shall 

 see, Weismann pictured development as a gradual process of 

 differential division, distributing the representative particles 

 or primary constituents of the germ-plasm. While this is 

 going on there is also a process of quantitative division, which 

 gives rise to the lineage of future germ-cells bearing the complete 

 equipment of germ-plasm, and this quantitative division also 

 occurs amid the qualitative divisions when many cells with 

 identical characters are produced. 



Criticism of Mosaic Theories. — ^These mosaic theories of 

 development have been criticised from many sides. Thus it is 

 pointed out that no visible phenomena of nuclear division 

 suggest that the partition may be qualitative ; on the contrary, 

 that the whole elaborate process of nuclear division seems 



