PS^-ON-TtSTMMETEICAL FORMS ""25^ 



a blastomere destined to produce in typical development only 

 one side of the body. Before proceeding further I should like 

 to point out the fact that the experimental production of two 

 whole embryos from one egg was successful only in those cases 

 where the first cleavage furrow separates the right and left ele- 

 ments of a bilaterally symmetrical animal, and not in those cases 

 where the first cleavage divided the dorsal from the ventral half a/ 

 "~1^I have indicated already in the "Festschrift fiir W. Roux" 

 the dorsal and ventral Anlage are incapable of restoring recipro- 

 cally one another. Those examples, where in the same species 

 the first cleavage furrow may either typically or atypically take 

 a different course, are extremely suggestive, for if a right half 

 is separated from a left half the reciprocal restoration is possible, 

 while dorsal or ventr al halves give ri seo nly to incomplete embryos. 

 We may recaTTm this connection Spemann's interesting experi- 

 ments on eggs of Triton and especially the atypical course of the 

 first cleavage in Ascaris-eggs induced by centrifuging in experi- 

 ments of Boveri and Hogue, also published in the Roux-Festschrift 

 1910, but not known to me at the time I arrived at the above 

 conclusion. 



y ' Considering now the results of regeneration in the light of the 



/ results of embryology, it would seem simplest to assume that the 



/ right and left sides of the body are not at all self-differentiated, 



\ but that they are conditioned by the other body-axes. In this 



I way the establishment of two axes each with two distinct poles 



/ (anterior and posterior; dorsal and ventral) would fully suffice 



{ to determine the bilaterality of the organism, because, if we 



\ presuppose a similar interaction of analogous anlagen, those 



\ lying in the third axis and giving rise to the right and left half 



\ of the body should naturally arrange themselves so that they 



I would represent mirror images of each other, when they occupy 



the same position in relation to the two differentiated axes. 



The problem of the determination of axes was also put forward' 

 with great clearness by Roux. He contrasted the differentia- 

 tion of the right and left body-side, induced by the meridian of 

 fertilization, with the predetermined and readily observable 

 dorso-ventral differentiation of the frog's egg. Inasmuch as the 



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