238 THE BIOLOGICAL BASIS OF INDIVIDUALITY 



develop as separate embryos, or they become secondarily integrated into a 

 single organism through the action of regulating mechanisms,— presumably 

 similar to those which are effective as contact and distance substances, — on 

 cells derived from the same embryo, and then an orderly development may fol- 

 low. In addition there may be observed certain intermediate conditions in 

 which the greater parts of the two embryonal structures remain distinct, but 

 some organs unite and become common to both organisms. Furthermore, under 

 certain conditions the joined organisms may separate again secondarily, the re- 

 sult of a process which may be designated as disagglutination. Structures rep- 

 resenting various stages of embryonal development can thus be united, un- 

 fertilized or fertilized eggs as well as early cleavage stages up to blastulae, and 

 perhaps even still farther advanced embryos. 



If two embryos have in this way been joined together by means of agglu- 

 tination into a single organism, giant individuals may develop, in which the 

 number of cells composing the embryo is approximately doubled, but in which 

 the size of the cells remains unchanged. However, in other cases in which, 

 at a very early stage, coalescence takes place between the two partners, an 

 organism with the same number but with double the size of cells results. 

 Under certain conditions it may happen that one of the two organisms 

 becomes atrophic and then the remaining parts of it may be dominated by 

 the larger partner. Such a dominance of a larger over a smaller partner has 

 been noticed repeatedly in cases of transplantation in lower invertebrates, as 

 well as in parabiosis in mammals. Whether the two embryos will form one 

 single organism or separate into two organisms depends upon several factors : 

 (1) The degree of development of the embryos and the rigidity of their tissue 

 and organ differentials at the time of union ; in general, the further the 

 embryonal development has progressed, the more the original plasticity of 

 tissues and organs has been lost, the less will be the chance that one single 

 individual will result from the union. (2) The orientation of the two surfaces 

 which unite the two partners; if this orientation is favorable then the union, 

 whether by means of agglutination or of coalescence, can be more readily 

 accomplished and a secondary separation becomes more improbable. This 

 conclusion agrees with observations in lower adult invertebrates, where the 

 covering of wound surfaces in the right orientation prevents regeneration at 

 the cut ends, but where the joining together of two unsuitable poles may 

 lead to budding or regenerative outgrowth and subsequent separation of the 

 component parts. It agrees in general also with the changes which take place 

 at the point of union between tissues in higher animals, where certain contact 

 differentials determine whether a stable or an unstable equilibrium will be 

 reached. (3) The result also depends upon the organismal differentials of the 

 two partners. Syngenesious and homoiogenous combinations apparently suc- 

 ceed. However, in many cases it is impossible from the reports of the investi- 

 gators to determine whether, in a certain experiment, a syngenesio- or a 

 homoiotransplantation was carried out, and we can therefore not be sure 

 whether any difference existed between the results of these two types of 

 transplantation; but some investigators, and in particular Bierens de Haan, 



