240 INTERNAL FACTORS IV. i 



E. G. CONKLIN. Mosaic development in Ascidian eggs, Journ. Exp. 

 Zoot. ii, 1905. 



H. DKIESCH. Von der Entwicklung einzelner Ascidienblastomeren, 

 Arch. Ent. Mech. i, 1895. 



H. DRIESCH. Ueber Aenderungen der Regulationsfiihigkeiten im 

 Verlauf der Entwicklung bei Ascidien, Arch. Ent. Mech. xvii, 1904. 



11. GENERAL CONSIDERATIONS AND CONCLUSIONS TO BE DRAWN 

 FROM THE FOREGOING EXPERIMENTS. 



From the foregoing experiments certain general conclusions 

 may now be drawn. 



(1) First, the division of the nucleus during segmentation at least 

 is not the qualitative process imagined by Roux and Weismann. 

 The pressure experiments of Hertwig, Driesch, and Wilson have 

 clearly demonstrated that the nuclei of the segmenting ovum 

 may be disarranged without prejudice to the subsequent normal 

 development of the embryo ; and this result is amply confirmed 

 by the normal behaviour of eggs (of sea-urchins) in which the 

 mutual positions of the blastomeres, and therefore also of the 

 nuclei, are altered, as well as by the very numerous cases in which 

 it has been shown to be possible to rear a perfect embryo or larva 

 from an isolated blastomere or blastomeres. 



It is evident that during segmentation at least the nuclei are 

 equipotential, and the hypothesis of self-differentiation in the 

 form originally propounded by Roux can no longer be upheld. 

 It has, in fact, been now abandoned by its author. 



(2) But though in this direction its labours have ended 

 negatively, modern experimental research is yet able to point to a 

 positive achievement of the greatest value and significance. For 

 the same series of investigations has shown that the cytoplasm of 

 the undeveloped ovum is not the homogeneous and isotropic 

 substance which the experiments of Pfliiger led Roux to consider 

 it, but heterogeneous, containing various specific organ-forming 

 stuffs which are definitely and necessarily connected with the 

 production of certain parts or organs of the developing animal. 

 If the polar lobe of the mollusc llyanassa be removed the larva 

 has no mesoderm ; if the polar lobe of Dentalium be taken away 

 the larva has no trunk and no apical organ ; the egg of a Ctenophor 



