i6o Regeneration 



ence of such '* organ-forming" substances (which may 

 or may not be specific) and of their flow in definite 

 channels explains the inhibitory influence of the whole 

 on the parts as well as the unbridled regeneration of the 

 isolated parts. 



2. We have seen that the resting egg can be aroused 

 to development and growth by substances contained in 

 a spermatozoon or by certain other substances men- 

 tioned in the preceding chapter. We will assume that 

 plants contain a large number of cells or buds which are 

 comparable to the resting egg cell, but which can be 

 aroused to action by certain substances circulating in 

 the sap; and that the same is effected for animal cells 

 by substances in the blood. In plants the cells which 

 can be aroused to new growth have very often a rather 

 definite location while in lower animals they are more 

 ubiquitous. For experimental purposes organisms 

 where these buds have a definite location are more 

 favourable, since we are better able to study the 

 mechanism underlying the process of activation and 

 inhibition (correlation). When a leaf of the plant 

 Bryophyllum calycinum is cut off and put on moist sand 

 or into water or even into air saturated with water 

 vapour, new plants will arise from notches of the leaf. 

 This is the usual way of propagating the plant and in no 

 other part of the leaf except the notches will new plants 

 arise. These notches therefore contain cells comparable 

 to seeds or to unfertilized eggs or to the mesenchyme 



