92 THE PHYSIOLOGY OF TWINNING 



Tornier has performed a number of experiments on 

 the salamander, Triton, involving artificial doubling of 

 tails and limbs, using the methods of constriction and 

 cutting off of parts. This mechanical isolation of grow- 

 ing regions results in the same types of doubling of limbs 

 and tails as does physiological isolation in other forms. 



Fig. 44. — Types of double frog embryos, due to inverting the eggs 

 in the two-cell stage. (After Schultz.) 



Spemann and Falkenberg (1920) have recently pre- 

 sented some very interesting data concerning the arti- 

 ficial production of twins in Triton. Early gastrula 

 stages were cut in two along the sagittal plane so as to 

 make two equivalent right- and left-hand half-embryos. 

 These as a rule regenerated the lost half, giving rise to 

 artificial monozygotic twins. About half of the right- 

 hand pieces showed situs inversus viscerum, or reversed 

 symmetry of the heart and stomach. This interesting 



