110 Phenomena Refuting Simple Epigenesis 
density.7!_ And these peculiarities make it certain that 
the relation of the isolated blastomere to the yolk plasm 
is not different from that which would have existed had 
it remained united to the other blastomere. 
But the preponderance of the determinative action of 
the yolk is limited usually to the early stages preceding 
the gastrula. It has no influence whatever on the final 
form of the embryo, no more than has for example, the 
temporary compression of the blastomeres between two 
plates or the shuffling of them. Therefore the early 
cleavage stages have no specific morphological signi- 
ficance, as is evident also from the above mentioned fact, 
that different related species and even quite widely 
separated species can present almost identical cleavage 
systems.7? 
It follows from this that as soon as development 
commences to take on its really specific form, that indi- 
cates that the action of the germ substance is preponder- 
ating over the action of the yolkplasm no matter in what 
way the latter may formerly have acted. 
Consequently simple epigenesis certainly cannot have 
recourse, in order to explain the half embryos of Roux, 
to the fact that the deutoplasm remained unchanged in 
the unsegmented blastomere. For these half embryos 
arise at very advanced stages of development and 
represent quite specific formations. 
Nevertheless Driesch seems to want to explain the 
half-formations in this way: “Each particle of the sur- 
™H, E. Crampton, Jr.: Experimental Studies on Gasteropod De- 
velopment. Arch. f. Entwicklungsmech. d. Organismen. Bd. 3; 
Heft. 1. Leipzig, Engelmann, March 24, 1806. 
"Cf. E. B. Wilson: The Cell-lineage of Nereis. Journ. of 
Morph., Vol. VI, No. 3. Boston, U. S. A., Ginn, July 1892. P. 455. 
