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.” ®* 
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.” %4 
“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. 
