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 plastic 

 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." 65 



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. 265266. 



