THE LENS-PROBLEM. 233 



epidermis was beyond doubt. What is responsible for the origin 

 of these lenses? Mencl suggested (p. 337) that "der, diese 

 zwecklose, wie durch Erinnerung der Epidermiszellen auftauch- 

 ende Linsenbildung auslosende Faktor ist die Vererbung." 



A much more reasonable and, I think, the only correct, inter- 

 pretation of this case was offered by Spemann ('03) who con- 

 cluded (p. 464) that the optic vesicles "oder genauer ihr fiir die 

 Linsenbildung allein in Betracht kommender retinaler Teil nur 

 scheinbar fehlen, indem die Partie der Hirhwand, welcher die 

 Linsen angelagert sind, nichts anderes ist, als die nicht abgeg- 

 liederte . . . Retina." 



Although this interpretation was later abandoned by Spemann 

 "unter dem Druck neuer Tatsachen" ('12, p. 3), my experience 

 with teratological material and Mend's case is a teratological 

 one forces me to lend it unreserved support. 



For the sake of clearness in presenting my point of view (based 

 largely on teratological data) the following brief recapitulation 

 of the present stage of our knowledge on the morphogenesis of 

 monsters may be permitted. 



It was repeatedly noted by pathologists and teratologists and 

 notably also by Mall ('09) that examination of malformed em- 

 bryos often discloses evidence of destruction, dissociation and 

 shifting of tissues or parts of the embryo. On examination of a 

 great many experimentally produced monsters in Fundulus 

 heteroclitus I ('i6a and b) was able not only to confirm these 

 observations, but also to demonstrate some remarkably striking 

 cases of the effects of such dissociation of parts of the early em- 

 bryonic primordium and to account for the causes of this process 

 which I have termed blastolysis. This "blastolytic action of the 

 chemically modified environment is ... a morphogenetic prin- 

 ciple common to all terata. . . . Blastolysis either destroys part 

 or all of the germ's substance, or it may split off and disperse 

 parts of the latter" (Werber 'i6b, p. 569). 



It is this destruction of tissues, the subsequent elimination of 

 parts destroyed and the resulting dissociation and shifting of 

 parts surviving that in experiments in which eggs are subjected 

 to a chemical modification of the environment, brings about the 

 weirdest malformations of the developing embryos. 



