EMBRYONIC RECONSTITUTIONS 543 



reconstitutions provide further evidence in support of the view that the 

 effective factors in the developmental patterns of early stages must be 

 sought in the dynamics of living protoplasms rather than in concentra- 

 tions of hypothetical formative substances. As we pass from later to ear- 

 lier stages of embryonic development, there is progressively less evidence 

 of regional specificity, and in some eggs at the beginning of embryonic 

 development there is apparently little or nothing more than quantitative 

 gradient pattern. In others regional specificities are already present at 

 the time of the first cleavage, though evidences of gradient pattern may 

 still be present. Some of these, in which the original pattern tends to 

 persist with little or no change in isolated parts and development appears 

 to be more or less completely a mosaic of independent, self-differentiat- 

 ing systems, are discussed in the following chapter. 



