UNION OF FREE-LIVING CELLS 291 



trorhiza, a foraminifera, E. Schultz found that the pseudopods of two sepa- 

 rate individuals can fuse with each other, but it is not certain whether this 

 represented a syngenesio- or a homoio-reaction. Similarly in Radiolaria, 

 Verworn (1892) succeeded in exchanging the nuclei between different, non- 

 related individuals of the same species. Such organisms remained apparently 

 normal and were protected by the possession of a nucleus of homoiogenous 

 character against the injurious effects which otherwise would have followed 

 loss of the nucleus. 



The extensive studies of Jennings, Sonneborn, and others, make it very 

 probable that genetic factors play a role in the mating reactions in Para- 

 maecium bursaria and aurelia, and similar conditions have been observed in 

 the green algae, Chlamydomonas, and other flagellates, by Moevus. In Para- 

 maecium bursaria, Jennings observed that conjugating pairs can be obtained 

 from mixtures of two appropriate clones, but not from either culture sepa- 

 rately. Within a few seconds after mixture the individuals have agglutinated 

 into small groups. If pairs of two agglutinated individuals form, the partners 

 in each pair are derived one from each of the two clones. These mating re- 

 actions occur provided certain physiological conditions, such as temperature, 

 light and state of nourishment of the Paramaecium are suitable, and the 

 agglutination takes place if two clones of, different reaction types are mixed. 

 But in certain clones, isolated pairs may be observed even between members 

 of the same clone, if the clone is left for a long time in a state of declining 

 nutrition ; the latter favors agglutination and this environmental factor may 

 overcome conditions inherent in the constitution of the different individuals. 

 A segregation into two different mating types may occur in some cases at 

 the first division after conjugation; in other cases, all clones descended from 

 the same pairs may represent at first the same reaction type and a segrega- 

 tion may take place only at later fissions. The meeting of two mates is acci- 

 dental, but an effective agglutination occurs only if the organisms belong to 

 different and suitable mating types. These two individuals remain united for 

 24 to 30 hours and during this time they exchange half of their chromosomes ; 

 however, there is no distinction between males and females in the sense that 

 one family would consist of males and the other family of females. After 

 separation, each parent multiplies by fission. The offspring is at first immature 

 and has not yet acquired the ability to undergo an effective agglutination with 

 an appropriate mate, but in the course of months they become mature. The 

 offspring of two parents that mated are all of the same type, which is usually 

 one to which one of the parents belonged ; but in some instances, they may 

 belong to another type. It seems, then, that it is not solely the genetic constitu- 

 tion which determines the mating type. 



Likewise, in experiments of Sonneborn, which preceded the ones just 

 mentioned, inheritable differences in mating types were observed in Para- 

 maecium aurelia. Here, in various stocks, collected in different localities, six 

 mating types could be distinguished, namely, types I, II, III, IV, V and VI. 

 Mating occurred only between types I and II, between III and IV, and 

 between V and VI. These three mating groups do not mate with one another ; 



