2 Edmund B. JVilson. 



portance of the cytoplasmic factors of localization and differen- 

 tiation was early recognized by Whitman in his remarkable paper 

 on Clepsine ('78) and emphasized by him in later papers. Sim- 

 ilar views were more or less clearly expressed by Van Beneden, 

 Flemming, Platner and others prior to the definite formulation of 

 the mosaic-theory of development by Roux in 1888.^ Roux 

 himself recognized from the first, as a prominent factor in his 

 theory, the importance of a definite topographical grouping of 

 specific cytoplasmic materials in the unsegmented egg; though 

 unfortunately this was complicated, then and in later discussions, 

 by the hypothesis of qualitative nuclear division, which has since 

 been shown to be untenable and has now been relinquished by its 

 author (Roux, 1903). Since that time the evidence, both cyto- 

 logical and experimental, has steadily increased that a prelocaliza- 

 tion of the morphogenic factors in the cytoplasmic regions is a 

 leading factor in the early development; and it has become evi- 

 dent that this is true not only in such "mosaic eggs" as those of 

 mollusks or ctenophores, but even in those of echinoderms or 

 nemertines, where an isolated blastomere or an egg-fragment may 

 produce a perfect dwarf embryo. It has become of high im- 

 portance to determine experimentally in what degree such pre- 

 localization or cytoplasmic "organization" may exist in the un- 

 segmented egg, and to what extent it may vary in different forms. 

 It is even more important for our general conception of develop- 

 ment to determine by the same method whether the prelocaliza- 

 tion of the morphogenic factors, in whatever degree it may occur, 

 exists from the beginning, or whether, as the cytological evidence 

 seems to show, it is established by a progressive process; for in 

 the latter case, as is hardly necessary to point out, prelocalization, 

 even in the unsegmented egg, may be brought under the category 

 of epigenetic phenomena ("epigenetic qualities" as distinguished 

 from "preformed qualities"^), and falls into harmony with hy- 

 potheses that assume the nucleus to be the primary determining 

 factor. 



The present studies, which are a continuation of the preceding 



1 Cf. my work on The Cell. 



2 Boveri ('03 ), p. 356. 



