258 HANS TRZIBRAM 



sions and also do not show the normal differentiation of the right 

 and left body-side. 



3. In monstrosities of this kind which are already well devel- 

 oped, both divisions of the appendage present the full character 

 of a crusher.^ 



4. If, on the contrary, the monstrosities are not yet com- 

 pletely developed, they always show transitional stages between 

 the nipper and crusher claws (figs. 10 and 11.) 



From these results, which are corroborated by instances from 

 all other heterochelous crustaceens, we may, I believe, draw the 

 conclusion, that monstrosities with an inverted symmetry are 

 not caused by the appearance of determinants of an opposite 

 body-side, but b}^ the inverted growth of the anlage of the same 

 body-side. As we have no reason to attribute the quite analo- 

 'gous monstrosities of appendages with inverted symmety occur- 

 ring in strictly bilaterally symmetrical animals to causes different 

 from those which are responsible for minor asymmetries, we may 

 therefore extend the above conclusion also to all bilaterally 

 symmetrical animals. 



II 



Now, let us turn from the province of regeneration to that of 

 embryology and consider the fundamental experiment of Roux 

 by which he was able to show that if one of the two first blasto- 

 meres of the frog's egg is injured, the remaining one was able to 

 develop alone and to produce a half-embryo representing either 

 the left or the right side of the organism. Thus with this expe- 

 Iriment Roux proved the possibility that the first two blastomeres 

 /give rise respectively to the left and to the right part of the body 

 ^in typical development. Further experiments however, by Roux 

 and others, soon demonstrated in the case of the frog as well as 

 of other animals the possibility of an atypical development, 

 where from one of the first two blastomeres a complete embryo 

 may arise, both sides of the embrj^o being thus produced from 



^ A good case of this kind lias been photographed recently by Leon J. Cole, 

 Biological Bulletin, vol. 18, 252, 1910. 



