CRITIQUE OF THE ROUX-WEISMANN THEORY 309 



wcuts from the beginning like an entire ovum of diminished size (Figs. 

 133, 124). The same result has since been reached by Morgan in the 

 teleost fishes, and by Zoja in the medusae. The last-named experi- 

 menter was able to obtain perfect embryos not only from blasto- 

 meres of the two-cell and four-cell stages, but from eight-cell and 

 even from sixteen-cell stages, the dwarfs in the last case being but 

 jig- the normal size ! 



These experiments gave a fatal blow to the entire Roux-Weismann 

 theory ; for the results showed that the cleavage of the ovum does not 

 in these cases sunder different materials, either nuclear or cytoplasmic, 

 but only splits it up into a number of similar parts, each of which 

 may give rise to an entire body of diminished size. 



The theory of qualitative nuclear division has been practically 



B 



Fig. 135. Modification of cleavage in sea-urchin eggs by pressure. 



A. Normal 8-cell stage of Toxopneustes. D. Eight-cell stage of Echinus segmenting under 

 pressure. Both forms produce normal Plutei. 



disproved in another way by Driesch, through the pressure-experi- 

 ments already mentioned at p. 275. Following the earlier experiments 

 of Pfliiger and Roux on the frog's egg, Driesch subjected segmenting 

 eggs of the sea-urchin to pressure, and thus obtained flat plates of 

 cells in which the arrangement of the nuclei differed totally from the 

 normal (Fig. 134) ; yet such eggs when released from pressure continue 

 to segment, without rearrangement of the nuclei, and give rise to per- 

 fectly normal larvae. I have repeated these experiments not only with 

 sea-urchin eggs, but also with those of an annelid (Nereis), which yield 

 a very convincing result, since in this case the histological differentia- 

 tion of the cells appears very early. In the normal development of 

 this animal the archenteron arises from four large cells or macro- 

 meres (entomeres), which remain after the successive formation of 

 three quartets of micromeres (ectomeres) and the parent-cell of the 



