SIX IN PROro/OA 



!43 



Faoi.uiion ok Q)NjL'(;Ari()N 



\Mien one tries to compare conjugation with other forms of 

 svnijamv, one is struck by the fact that in typical examples most of 

 tlie potential gametes fail to develop. Since three pregamctic divisions 

 in each coniufrant seem to he the rule, potentially eight gametes could 

 be produced from each original micronuclcus, whereas normally only 

 two become functional. This suggests that bringing the gamonts (con- 

 jugants) togetlier reduces the hazards to potential gametes, since an 

 exchange of gametes can be effected with dangers of loss greatly 

 reduced. Conjugation might therefore be thought of as an evolu- 

 tionary product derived from more primitive ancestral conditions 

 where a brood of gametes was produced, somewhat like that in 





-QD ) (ccg^ 13 <^^0 _ _ 



C> (k> O <D> "CD (2> <£> (S) 



Fig. am. Scheme of life cycle of Dallasia frovtata including gamete brood 

 formation, from Calkins and Bowling (1928), redrawn. 



1, 2, young "tailed" forms; 3-5, division of "tailed" forms; 6, preconjugant; 7, con- 

 jugation of "tailed" forms; 8, "boat" form, 9-11, divisions without growth, to pro- 

 duce gametes; 12, encystment of sister gametes; 13, 14, zygote formation; 15, 16, 

 excystment and development of new "tailed" forms. 



