INDIVIDUALITY OF EMBRYONAL TISSUES 241 



under these conditions a double set of chromosomes as well as twice the 

 amount of egg substances. Such eggs could then be fertilized and could de- 

 velop into one individual. In case two already fertilized eggs were united, the 

 two female and also the two male nuclei, respectively, united with each other, 

 and either twins or single embryos resulted from such combinations. If, instead 

 of eggs, early embryonal stages were combined, they tended to agglutinate 

 rather than to undergo coalescence, the protoplasm of the individual cells 

 remaining separate. However, the direction of the axes of the two organisms 

 was found to be of significance; if there was correspondence of direction, 

 one complete organism developed, otherwise two separate individuals. In 

 general, the two factors which above all others seem to be of importance in 

 such combinations and which determine whether one or two organisms shall 

 be formed from two young embryos are : (1) The developmental stages of the 

 embryonal structures, and (2) the direction of the axes of the organisms. 



As to the effects of the organismal differentials, Bierens de Haan found 

 that different combinations succeeded unequally well, it being easier to unite 

 certain species than others. Heterogenous fusions succeeded only rarely, and 

 if they did succeed, the resulting fusion was less complete. In this case there 

 was a chance for a secondary separation of the partners, similar to the separa- 

 tion which had been observed if two more distant species were joined together 

 in primitive invertebrate adult organisms and in amphibian embryos. Separa- 

 tion could still occur as late as after eighteen hours ; but if in other cases these 

 heterogenous combinations developed, the development was not quite typical 

 and it was slower than normal. Unified, single giant plutei never resulted from 

 such heterogenous unions. Some combinations of this kind, however, succeeded 

 better than others and in these successful experiments the intestines were seen 

 to grow from one into the other partner. But even in relatively successful 

 heterogenous transplantations, such as those between Parechinus and Para- 

 centrotus, the organ formation was rudimentary and development soon ceased ; 

 perhaps substances which were produced as the result of incompatibilities 

 between the two organisms acted as poisons. In a combination between Pare- 

 chinus and Paracentrotus the dominant Parechinus could call forth the de- 

 velopment of a line of cilia at the expense of the rudimentary Paracentrotus, 

 and in this case parts of the skeleton seemed to act as organizers. In general 

 it was found that heterogenous combinations never led to the formation of 

 really uniform organisms, but that at best merely sectorial chimaerae were 

 produced. Under the most favorable conditions each component of these com- 

 binations could develop to the stage of a normal pluteus, otherwise regulative 

 processes occurred which led to separation ; in other cases, both heterogenous 

 partners became sickly. As far as unfertilized ova are concerned, it was ob- 

 served that while a homoiogenous union could be accomplished, heterogenous 

 unions did not succeed. From the latter, unified single giant plutei never 

 formed; in some instances cytolysis, in others a temporary agglutination oc- 

 curred, but a real coalescence did not take place. However, even twin larvae 

 which developed from a homoiogenous union often showed some defects. We 



