H. V. BR0NDSTED 



If the shift, however, has been severe, the half blastema with the high regeneration 

 rate will have time enough to regenerate not only its own eye but also the symmetrical 

 one, because no inhibiting force has been set up yet in the other half; this other half 

 is under so strong an inhibitory force that, on account of its very slow regeneration 

 rate, it has no chance whatever to differentiate eyes. 



Between these two conditions there must be one in which the relations between the 

 two halves of the blastema are such that both of them may get time enough to make 



A 



Figure 18. Planaria lugubris. 

 Two halves reunited with 

 moderate shift. A transverse cut 

 was made in the foremost part 

 of the right piece hitting the field 

 at a high level, but in the left 

 half hitting the field at a some- 

 what lower level. 



Figure 19. Planaria lugubris. 

 The two-headed chimera result- 

 ing from the operation indicated 

 in Figure 18. 



not only their own eye but also a symmetrical one before the inhibiting influence from 

 the other half has reached them. 



If an amphibian egg in the two-cell stage is transected into two separate cells, 

 provided the first cleavage furrow has taken place near the median plane, each of 

 them will regenerate the missing part, and twin embryos are formed. It is just the 

 same when a planarian is split longitudinally: two worms will come forth. 



It is also true that if one of two amphibian blastomeres is killed by cautery, but 

 otherwise left in its old position as a symmetrical part of the cleaved egg, the living 

 blastomere will continue its development into a half embryo. That is to say, the killed 

 blastomere contains inhibitory forces exercising their influence for a long period of 

 time. 



130 



