DOUBLE STRUCTURES 139 



Feet with supernumerary digits can also be produced by artificial 

 wounds. If the first and second and then the fourth and fifth toes 

 are cut off, as indicated by the lines in Fig. 43, E, so that a part of 

 the tarsus and a part of the tibia and fibula are cut away (the 

 third finger being left attached to the remaining middle portion), more 

 toes grow out from the wounded surface than were removed, as shown 

 in Fig. 43, F. A similar result may be obtained in another way. If 

 the first and second toes are cut off by an oblique cut (Fig. 43, G\ 

 and then after the wound has healed the third, fourth, and fifth toes 

 are also cut off by another oblique cut (a part of the tarsus being 

 removed each time), more toes are regenerated than were cut off 1 

 (Fig. 43, H\ 



Tornier suggests that the double feet that are sometimes formed in 

 embryos even in the mammalia have resulted from a fold of the 

 amnion constricting the middle of the beginning of the young leg, in 

 the same way as is brought about artificially by tying a string over 

 the growing end of the regenerating leg of triton. 



In many of these cases, in which the double structure is the result 

 of splitting the part in the middle line, the completion of the new 

 part is exactly the same as though the parts had been entirely sepa- 

 rated. The only special problem that we meet with in these instances 

 is that this doubling is possible while the piece remains a part of the 

 rest of the organism. This shows that there is a great deal of inde- 

 pendence in the different parts of the body in regard to their regen- 

 erative power, and that local conditions may often determine the 

 formation of double structures. 



It has been shown during the last decade that double embryos may 

 be produced artificially by incomplete separation of the first two 

 blastomeres. Driesch, Loeb, and others have demonstrated that if the 

 first two cells of the egg of the sea-urchin be incompletely separated, 

 each may produce a single embryo and the two remain sticking to- 

 gether. Wilson has shown in amphioxus that the same result occurs 

 if the first two cells are partially separated by shaking. Schultze has 

 shown in the frog that if at the two-cell stage the egg is held in an 

 inverted position, i.e. with the white hemisphere turned upwards, each 

 blastomere gives rise to a whole embryo the two embryos being 

 united, sometimes in one way, sometimes in another, as shown in Fig. 

 63. In this case it appears that the results are due to a rotation of 

 the contents of each blastomere, so that like parts of the two blasto- 

 meres become separated. In the egg of the sea-urchin, and of am- 

 phioxus, gravity does not have a similar action on the egg, but the 

 results seem to be due to a mechanical separation of the blastomeres. 

 These cases of double structures, produced by the segmenting egg, 



1 In the figure one double or forked toe is present. 



