104 Regulation of Size in Unicellular Organisms 



2. The Conditions for Conjugation 



It is difficult to distinguish some kinds of plasto- 

 gamy from true conjugation, for it is not certain that 

 every fusion of nuclei is a true union of gametes. It 

 has often been supposed that conjugation might be a 

 mechanical process, occurring whenever the superficial 

 cytoplasm became sticky. This view would require the 

 corollary that in species where an external condition 

 produced the stickiness, some mechanism caused the 

 nuclei of fused bodies also to fuse. 



The conditions for conjugation are partially known 

 in some species 2 . It is fairly certain that deep-seated 

 physiological changes accompany whatever physical 

 condition of the cytoplasm is requisite, at least in cases 

 of typical conjugation. 



Occurrence. The phenomena of conjugation, in 

 those species of ciliates upon which studies have been 

 made, lead to the following picture of its occurrence. 

 The ability to conjugate as gametes is, probably, 

 wholly absent in some species or clones. Where pres- 

 ent it can be entirely prevented by appropriate cultural 

 conditions. Where inducible, the conditions for induc- 

 tion differ not only for each species, but for clones 

 within the species. With proper means of induction, 

 conjugation can be induced very frequently, and there- 

 fore with little reference to any particular interval of 

 time since it last occurred. 



It is to be supposed that conjugation can occur in 

 any species where there is a morphological differen- 

 tiation of gametes, even though such conjugation has 

 not been seen. But the union of the gametes, of course, 

 need not occur under all conditions under which they 

 are formed. 



* Stein ('54), Everts (73), Butschli (76), Maupas ('89), Verworn ('00), 

 Enriques ('07, '08, '10 *16b), Woodruff ('08), Jennings ('10), Zweibaum 

 ('12), Hopkins ('21), Dogiel ('25), Ball ('25b), Austin ('27). 



