CHAPTER VII 



TWINNING IN AMPHIBIA, REPTILES, AND 

 OTHER CHORDATES 



AMPHIBIA 



Twinning appears to occur with extreme rarity among 

 the Amphibia. The fact that normal development takes 

 place at very low temperatures may partially explain the 

 failure to twin; for low temperature seems to be the main 

 reason for twinning among egg-laying vertebrates other 

 than the fishes. Twinning may readily be induced among 

 the Amphibia, however, by experimental procedures. 



0. Schultze (1894) showed in the case of the frog 

 that if the egg is inverted while it is in the two-cell stage 

 of cleavage in such a way that the white or vegetative 

 pole is turned upward, each blastomere will give rise to 

 a whole embryo. Figure 44, page 92, shows several of 

 his double embryos at various stages of development. 

 Morgan (1901) in commenting upon these results says: 

 ''In this case it appears that the results are due to a rota- 

 tion of the contents of each blastomere so that like parts 

 of the two blastomeres become separated." There is also 

 doubtless another reason for the physiological isolation 

 of the two hemispheres, viz., retardation, due to the long 

 delay consequent upon the necessity for each blastomere 

 to undergo a complete reorganization of its axiate struc- 

 ture. Each cell, after the temporary cessation of devel- 

 opmental activity, starts out for itself as though it were 

 a separate egg, and each produces a whole embryo. 



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