RELATIVE STABILITY OF LIVING MATTER 335 



That the new material produced in regeneration is at first 



totipotent that is, capable of more than one differentiation 



is easily shown. If a short piece be cut from the middle of a 

 planarian, and if, after the new material has begun to form, the 

 whole mass be split lengthwise, both halves will develop heads 

 directly; but if the split is not made " until just before the 

 formation of a head, then each half piece produces at first a 

 half head, that completes itself later at the cut side." 1 



Again, if the head be cut from a planarian and the body be 

 split for a distance, the split will heal and a single head will 

 regenerate ; but if a slice be taken out of the middle line of the 

 body, or if otherwise the two pieces be prevented from fusing, 

 then two heads will regenerate, one on each piece. 2 These two 

 heads may later pull apart with sufficient force to tear the body. 3 



All these phenomena reveal great capability of readjustment 

 as to more or less differentiated tissue. The more the matter is 

 studied the more we discover that the line between regeneration 

 and development is one of degree rather than of kind, and that 

 differentiation in both cases, whether normal or abnormal, rests 

 upon causes very imperfectly understood but closely akin to 

 those of differentiation in general. 



SECTION IX EVIDENCE FROM GRAFTING 



When tissue of one kind, plant or animal, is removed from 

 its connections and set into tissue of the same or of a different 

 kind, and a union takes place so that growth ensues, the union is 

 called a graft. From our standpoint no little interest attaches to 

 the variety of conditions under which grafts may be effected, and 

 to the fact that the growth upon the graft is like the part set in 

 and not like the tissue that supports it, the host acts only as 

 affording standing room and food supply to the graft. Again, if 

 two dissimilar pieces are joined, each preserves its identity. 



Grafting is comparatively easy among plants, though the species 

 that may be joined are limited. Among animals it is more diffi- 

 cult, but by no means impossible. 



1 Morgan, Regeneration, p. 49. 2 Ibid. p. 50. 



3 Loeb, Physiology of the Brain, p. 82. 



