CONJUGATION 81 



whicli has separated from its partner divides into two daughter ciliates, 

 each of which has a single macronucleus and two micronuclei, as in the 

 original conjugants. 



In the conjugation of Collinia branchiarum described below, the two 

 ciliates unite as inParameciu )ii , and exchange of nuclei takes place(Fig. 495). 

 The macronuclei, however, behave in a remarkable manner. Each becomes 

 much elongated, and when exchange of micronuclei is taking place, the 

 two long macronuclei arrange themselves side by side across the point of 

 union of the ciliates in such a manner that half of each macronucleus is in 

 each ciliate. When the ciliates separate the macronuclei divide, so that 



Fig. 46. — Conjugation of the Ciliate Cycloposthium bipalmatum, showing 

 Differentiation of Conjugating Nuclei into Male (^J) and Female ( 2 ) 

 ( X ca. 300). (After Dogiel, 1923.) 



M., Macronucleus ; Sk., skeletal plate ; An., anus; Ph., pharynx ; My., myonemes ; D., degenerating 

 micronuclei; Sp., remains of central part of spindle. 



each ciliate receives half of each macronucleus. Though this occurs, the 

 macronuclei ultimately degenerate, and a new macronucleus is formed 

 from the micronucleus. It will thus be seen that in the Euciliata each of 

 the two conjugants ultimately contains two nuclei which are exactly alike, 

 except that one is a migratory or male nucleus, and the other a stationary 

 or female nucleus. This difference in behaviour is the only indication of sex 

 differentiation. In the case of Cycloposthium hipalmatum, a ciliate parasitic 

 in the intestine of the horse, Dogiel (1923, 1925) has noted that, though 

 conjugation between two individuals takes place in the usual manner, the 

 two nuclei which take part in the syngamic process differ in that the 

 migratory one assumes the characters of a male gamete in becoming a 

 I. ■ 6 



