618 FERTILIZATION 



changed by conjugation into new genetic entities, but this is actually 

 genetic transformation rather than reproduction. Before the exconju- 

 gants return to their normal vegetative condition, they undergo one or 

 more divisions in most cases, but these divisions are reproduction by 

 binary fission, an asexual phenomenon. Although these divisions are 

 modified by the previous sexual union, they are none the less asexual 

 reproduction. 



Conjugation is peculiar to the Ciliata and the process is strikingly 

 uniform, with but few exceptions, in all ciliates which have been studied. 

 The general course of the maturation phenomena in conjugation was 

 first described by Maupas (1889), who studied the process in a number 

 of ciliates and divided it for convenience into eight stages. Calkins 

 (1933) states that "With one or two exceptions {Trachelcicerca phoene- 

 copterus, Spirostojnum amhiguum, etc.) all of the free living ciliates 

 thus far described agree in the general course of their maturation phe- 

 nomena." Several parasitic species, however, exhibit some important dif- 

 ferences from the usual course, and recent investigations have revealed 

 a few interesting deviations among free-living forms. 



With a few noteworthy exceptions, the union of two ciliates in con- 

 jugation takes place longitudinally and symmetrically (Fig. 146). The 

 first sign of approaching conjugation in a mass culture is frequently 

 a tendency to agglomerate in dense masses. Individuals appear to stick 

 together on contact, even though they may separate soon after. Even- 

 tually, two individuals will adhere side by side, or with ventral surfaces 

 together, and become more intimately connected in the anterior region. 

 The extent of union varies from a thin protoplasmic bridge at the time 

 of cross-fertilization to an intimate fusion of more than half the body 

 length in other species. 



Two individuals of Euplotes patella will come into contact, spiral 

 about each other for a few moments, and then apply themselves to- 

 gether at their left peristomal margins, so that the appearance is similar 

 to two turtles stuck together by their left ventral halves (Fig. 146) . They 

 swim forward together in a well coordinated manner, rotating on an axis 

 which, owing to the symmetry of the pair, is straighter than the spiraling 

 axis of a single individual. At this stage the pairs are joined only by 

 their cirri. After remaining together a short time, they may separate and 

 repeat the process with the same or with other individuals, until finally 



