380 The American Naturalist [May, 



embryo the old half may be regarded as exercising a sort of 

 formative or assimilative action upon the irregular mass of 

 yolk cells in the new half. 



To recapitulate the chief re- 

 sults with the aid of fig. 1, A, 

 B, C : A, being a half blastula 

 resulting from injury of one of 

 the first two cleavage cells and 

 as yet not reorganized, seen in 

 section: B, a right half lnrva 

 from dorsal view and C are 

 anterior-half larva. Injury to 

 one of the first cleavage cells 

 may result in the formation of 

 a half embryo, to one of the 

 first four to a fourth embryo, 

 to three of the first four to a 

 l three-fourths embryo. The 

 , dead cells (regarded as dead by 

 'the author) may be revived 

 and reorganized partly by di- 

 al of the nucleus present 

 and partly by inwandering of new nuclei and their division. 

 The nucleation is followed by a cellulation and this by a re- 

 generation of germ layers not following the normal course but 

 growing out from the exposed surfaces of the layers already 

 formed in the other half of the embryo. This definite process 

 of post-generation seems due to the controlling action of the 

 formed germ layers upon nuclei and yolk collected by chance 

 in the places they occupy before being thus incorporated with 

 the germ layers. Thus the reorganized parts are not capable 

 of self differentiation as are the early colls in cleavage but are 

 dependent upon influences coming from the other half of the 

 egg or embryo. 



An earlier paper by the same author, Roux, 1 has ledtointer- 



»W. Roux: II Beitrage zur Entwicklungsmechanik des Embryo jj 

 die Bestimmung der Haupt-richtung des Froschembryo im Ei und die erste lheiiuu 6 



BreslauerTr'ztliche Zeitschrifi, 7, 1SS5. 1. nrs, 64-68, 73-77, 87-88, lW-Wl, 



