Temporarily Disturbing Action of Yolk Mass 97 
These embryonal organs and modifications which in- 
terpolate themselves in the series of ontogenetic stages, 
leaving these latter unchanged, can then also serve as 
proof that developing organisms are elastic but not plas- 
tic, while contrariwise grown organisms remain piastic 
but not elastic. 
To these facts one can add that the large accumulation 
of yolk in the egg cells exerts a great influence on the 
first stages of development, but subsequently exerts abso- 
lutely no influence on the other stages. “The organiza- 
tion of the egg,” says Hertwig, “which depends on the 
disposition of the deutoplasm, has fundamentally only a 
subordinate influence, and that of a secondary and tran- 
sient nature in the developmental process.” “Eggs of ani- 
mals which belong to different races can present a very 
similar type of cleavage and similar early embryonic 
forms, while eggs from closely related divisions of one 
and the same race divide in very different ways, and dif- 
fer very extraordinarily in the nature of the blastula and 
gastrula. The deposition of yolk material in the egg im- 
prints a quite characteristic stamp upon the first embry- 
onic stages,—the cleavage process, the blastula, gastrula 
and so on,—but it has no influence on the essence of the 
animal species itself, nor on the formation of any special 
species of animal.’ ® 
One has here then developments, which, altered in 
the first stages by the influence exerted by the yolk mate- 
rial, later resume their normal course, exactly as though 
they had undergone no alteration. In other words the 
yolk substance alters the normal development only tem- 
porarily, only for so long as its action continues to make 
*'Oscar Hertwig: Die Zelle und die Gewebe. II. P. 265—266. 
