96 Hypothesis of Structure of Germ Substance 



ism of self -regulation, active during the whole of onto- 

 geny but ceasing upon the completion of development. 



Another example of the dynamic elasticity of devel- 

 opment, no less characteristic in certain respects than the 

 preceding, has been repeatedly observed by Roux in the 

 postgeneration of his half embryos. "In the postgenera- 

 tion of the mesoblast it can be observed that very young 

 yolk cells with nuclei not yet stainable, and also the re- 

 mains of substances not yet cellulized, hinder the differen- 

 tiation, and so divert the mesoblastic formation toward 

 the interior or divide the formation into two layers; but 

 after the circumvention of this obstacle the further dif- 

 ferentiation soon resumes its normal course ; a procedure 

 in its essence extremely puzzling." 63 



It may be merely noted here that this elasticity of de- 

 velopment helps to explain the interpolation of certain 

 newer ontogenetic formations or stages (placenta and 

 similar things) in the series of older ontogenetic stages, 

 without markedly altering the earlier or later members 

 or even the last member of this ancient series. 



"We have reason to believe/' says Orr, "that the man- 

 ner of growth for some particular period of the develop- 

 ment may be secondarily changed without radically affect- 

 ing either the preceding or succeeding growth. As an ex- 

 ample of this may be mentioned the embryonic organs and 

 embryonic modifications which adapt the embryo to 

 undergo partial development in the body of the parent, 

 and allow it to receive nutriment from the parent, e. g. 

 the placenta." 64 



es Wilhelm Roux: Uber die kiinstliche Hervorbringung halber 

 Embryonen usw. Virchows Archiv. Bd. 114, October 1888. P. 276. 

 Gesamm. Abhandl. Zw. Bd. P. 504. 



"Orr: A Theory of Development and Heredity. New York, 

 Macmillan, 1893. P. 210, 



