T. M. SONNEBORN 159 



been amply confirmed. Objections can also be raised against the 

 evidence for species crosses, but this needs to be reinvestigated 

 with the better methods now available. In any case, no viable 

 hybrids between species were claimed by these workers and 

 that, from our point of view, is perhaps the main thing. 



The modern work on Ciliates began with the discovery of mat- 

 ing types in P. aurelia (Sonneborn, 1937). As originally used, the 

 term mating types refers to physiological differences between 

 individuals that mark them off into mating classes. True conju- 

 gation does not ordinarily occur between individuals of the same 

 physiological class or mating type; it occurs between individuals 

 of complementary physiological classes or mating types. This is 

 the sense in which the term has come to be used by students 

 of Protozoa and Fungi. But it has a somewhat different 

 meaning in Euplotes, as will be explained later, and the possi- 

 bility that conjugation can occur between individuals of the 

 same mating type in certain clones of some species or in certain 

 stages of the life cycle has not yet been completely excluded. 

 These special cases will be dealt with as they arise. 



Commonly, but not invariably, the mating type of an individ- 

 ual is inherited by its asexually produced descendants. This 

 makes it possible to use clones rather than individual animals in 

 the study of breeding systems. Such studies quickly exposed 

 fundamental relations in P. aurelia (Sonneborn, 1938) and in 

 P. bursaria (Jennings, 1938) that have subsequently been widely 

 confirmed in sufficiently studied Ciliates in which mating types 

 have been found. If many collections are made from different 

 natural sources (ponds, lakes, streams, rivers, and the like) and 

 a strain from each source is established in the laboratory, the 

 general features of their breeding relations are ordinarily found 

 to be those described in the following paragraphs. 



Clones of the same strain are either of the same or of comple- 

 mentary mating types. When complementary, they conjugate 

 promptly and abundantly with each other under appropriate 

 conditions. When of the same mating type, they cannot be made 

 to conjugate with each other. Every clone of every strain has a 

 definitely assignable mating type. When mating types of any one 



