162 CHARLES R. STOCKARD 



5. EXPERIMENTAL PRODUCTION OF TWINS AND 'DOUBLE MON- 

 STERS' BY AN EARLY ARREST OF THE DEVELOP.MENTAL RATE 



One of the earliest accomplishments in experimental embiy- 

 ology was the production of two embryos, or twins, from a single 

 egg (Driesch, '92; Wilson, '93; Morgan, '93; Zoja, '95; Loeb, 

 '95; Schultze, '95, and others). This phenomenon was first pro- 

 duced by separating the two primary blastomeres so that they 

 were no longer in their usual intimate relation, each then devel- 

 oped independently and produced a complete individual. In the 

 light of this striking experiment, the occurrence of twins and 

 double monsters under natural conditions was readily explained 

 as being the result of an undue separation of the two blastomeres 

 during the first cleavage. Such a separation might have been 

 caused in a mechanical way, the two cells being pressed or 

 squeezed apart, or something unusual in the chemical nature of 

 the environment may have reduced the normal degree of cohe- 

 sion between the first two blastomeres, allowing them to fall 

 abnormally far apart and finally to become entirely separated 

 from one another. 



This clean-cut experimental production of twins and its ready 

 application and acceptance as an explanation of the modus oper- 

 andi for a well-known natural phenomenon, has undoubtedly 

 held back our real understanding of the phenomenon and strik- 

 ingly illustrates the dangers of directly interpreting occurrences 

 in nature on the basis of results from experiments. 



Almost at once evidence began to accumulate which questioned 

 the general application of the separate blastomere explanation of 

 twin formation. Such evidence was not always appreciated in 

 this connection, but from our present point of knowledge its 

 bearing is more readily seen. The discovery was very soon 

 made (Wilson, '04; Conklin,'' '05, and others) that on separating 

 the primary blastomeres in certain species of eggs complete twin 

 embryos do not result. Yet there is no reason to believe that 

 in nature twins and double monsters do not at times arise from 

 the eggs of such species. Twin formations are certainly not due 

 to the separation of the first two blastomeres in these particular 

 species, since each of these blastomeres developing independently 



