ORIGIN OF MONSTERS. II 411 



I regard destruction of parts of the primordium by chemical 

 action and fragmentation resulting from the latter as well as 

 from differences between the osmotic pressure in the eggs and 

 that of their surrounding, experimentally modified, medium. 



These conclusions can, I think, be extended to the morpho- 

 genesis of 'monstra per excessum.' For Driesch ('93) has 

 demonstrated for the sea-urchin and Wilson ('93) for Bran- 

 chiostoma that the development of various 'monozygotic' 7 

 double or multiple embryos can be induced by the employment 

 during the first or the second cleavage of a dissociating force 

 such as raising the temperature or shaking the eggs, which 

 would separate the blastomeres. Similar results were obtained 

 also by Fischel ('98) by mechanical pressure exerted on the eggs 

 of the ctenophore Beroe ovata. It was further shown by Loeb 

 ('95) that like results can be obtained in sea-urchin eggs trans- 

 ferred soon after fertilization to a medium of lowered osmotic 

 pressure. 



Of particular interest for our consideration is also the work of 

 Bataillon ('01) who produced duplicities experimentally in lower 

 vertebrates (Petromyzon and the teleost Leuciscus rutilus) by 

 increasing the osmotic pressure of the surrounding medium. 

 Employing this method Bataillon was able to ascertain by direct 

 observation in Petromyzon that the development of monozygotic 

 twins and various other duplicities resulted from the more or less 

 complete separation of the blastomeres of the two-celled stage. 

 He also states that in Petromyzon at least one-third of the egg 

 seems to be necessary for the development of a whole embryo. 



This equi- and totipotency of the first blastomeres was dem- 

 onstrated also in the amphibians. Thus O. Schultze ('94) and 

 Wetzel ('95 and '96) have succeeded in producing conjoined double 

 embryos in Rana fusca by inverting for some length of time 

 eggs in the two-celled stage, previously compressed between two 

 glass plates. 



Even complete monozygotic twins (of half the normal size 

 may be obtained in amphibians by separating the blastomeres, 

 as shown by the ingenious experiments of Herlitzka ('95 and 



7 This very precise term is adopted from Newman ('17). 



