REGENERATION AND TISSUE EQUILIBRIUM 283 



from the blastomeres ; this is a problem which we have already discussed in 

 a previous chapter. Developmental processes which might lead to the forma- 

 tion of two embryos are prevented if the surfaces of the blastomeres, either 

 derived from the same or from different eggs, are oriented to each other in 

 the right direction and if the organismal differentials of the joined parts are 

 mutually compatible. Under these conditions adjoining cells, even if they were 

 obtained from different organisms, may restrain each other from carrying 

 out movements and from undergoing cell divisions, such as would give rise 

 to the formation of a whole organism from one of the partners; instead, the 

 blastomeres may coordinate the activities of the neighboring cells with their 

 own. 



However, if the organismal differentials of the partners are unsuitable, or 

 if the axes of the adjoining segments do not fit each other, then the neighbor- 

 ing segmented cells no longer exert this regulating effect. When unsuitable 

 heterodifferentials cause the duplication of organisms, the two partners may 

 still remain united in a mechanical sense; but sometimes a complete separa- 

 tion occurs. Conversely, in the normally segmented ovum each blastomere may 

 develop into a separate individual if the surfaces through which the blasto- 

 meres are joined are altered, or if the substances lying at the surfaces of the 

 cells are made to move. The same conditions in the surrounding medium which 

 prevent the spontaneous separation of joined together blastomeres and the 

 subsequent initiation of abnormal growth processes, may also bring about the 

 union of two organisms into one. In regeneration in both adult and in em- 

 bryonal tissues the character of the organismal differentials, the nature, and 

 in particular, in certain cases, also the orientation of the parts of cells or 

 tissues adjoining each other, determine whether or not movements of cells, 

 as well as cell multiplications, shall be initiated, which may lead to the forma- 

 tion of separate organisms; in the case of the ovum, movements of special 

 substances also play a role in this regard. 



In general, transplantation of suitable tissues onto remnants of embryonal 

 tissues prevents regeneration of the host embryonal tissue, and conversely, 

 the latter may prevent such growth in the transplant; but if various incom- 

 patibilities exist, these act as stimuli which may cause an outgrowth from the 

 host or a duplication of the transplant. Such incompatibility may consist in 

 differences in organismal differentials or in the contact of otherwise unsuit- 

 able tissues; even the turning around of a longitudinal axis of one of two, 

 ordinarily suitable, tissues may bring into contact unsuitable tissues. But, an 

 embryonal bud does not tend to reduplication if the strangeness of the soil 

 onto which it is transplanted exceeds a certain limit of unfavorableness. Thus, 

 if limb buds are transplanted to the head or medulla of larvae of salamander, 

 conditions which favor duplication are lacking. 



We find, therefore, that very early embryonal buds of amphibia behave in 

 a similar manner to adult organisms of very primitive classes of animals ; 

 also, that very young embryonal material and early regenerative stages in 

 adult primitive animals behave very much alike. In all these cases, we have 

 to deal with plastic material, where a certain degree of unsuitability between 



