536 PATTERNS AND PROBLEMS OF DEVELOPMENT 



EMBRYONIC DUPLICATIONS AND POLYEMBRYONY 



Differential inhibition of embryonic stages not infrequently results in 

 duplication or multiplication of parts or axes and sometimes in develop- 

 ment of more than one individual from a single egg or early embryo. Ex- 

 amples are the duplications in insect development resulting from differen- 

 tial inhibition by cyanide (p. 518), polyembryony in fishes after inhibition 

 by low temperature (Stockard, 192 1) and by exposure to ultra-violet light 

 (Hinrichs and Genther, 193 1), and dupHcations in chick embryos resulting 

 from various inhibiting conditions. Partial duplications in annelids and 

 other forms may also result from inhibiting conditions. In these cases 

 there is apparently a decrease in dominance with physiological isolation of 

 regions normally subordinate. Temporary inhibition sufficient to prevent 

 rapid recovery of the original dominance followed by return to natural 

 conditions, permitting parts originally subordinate to become dominant, is 

 apparently most effective in producing these duplications. 



Duplications and multipHcations, ranging from bifurcations, through 

 all degrees of teratological duplications and multiplications and complete 

 twins from single eggs, to development of many, even hundreds, perhaps 

 thousands of embryos from a single egg, occur without experimental inter- 

 ference. Except in a few forms, complete and partial twinning appear only 

 occasionally, as do the teratological forms, and many of them probably re- 

 sult from inhibiting conditions. The extreme types of polyembryony are 

 usually normal characteristics of the species concerned. The natural 

 polyembryonies raise interesting questions concerning developmental pat- 

 tern, but at present it is possible in most cases only to call attention to 

 some of them and to suggest possibihties. 



Polyembryony has been observed in various coelenterates as a conse- 

 quence of separation of blastomeres and blastomere groups, a "blastomere 

 anarchy," as Metschnikoff describes it.'^ These cases may be results of 

 slightly inhibiting laboratory conditions or other unfavorable conditions. 

 Low oxygen in standing water may be sufficiently inhibitory to obliterate 

 any pattern originally present in early embryonic stages, with resulting 

 isolation of cells or cell groups, and the differential between free surface 

 and surface in contact may determine new polarities in the isolates, or they 

 may reconstitute from the part of the original pattern persisting in them. 



In several genera of bryozoa the blastomeres of earlier stages are ap- 

 parently completely separated, and follicle cells may lie between them. 



"See, e.g., Busch, 1851; Haeckel, 1881; Metschnikoff, 1886. 



