no Phenomena Refuting Simple Epiyenesis 



density. 71 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. 72 



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- 



71 H. E. Crampton, Jr. : Experimental Studies on Gasteropod De- 

 velopment. Arch. 1 Entwicklungsmech. d. Organismen. Bd. 3; 

 Heft. i. Leipzig, Engelmann, March 24, 1896. 



72 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. 



