178 



PRINCIPLES OF ANIMAL BIOLOGY 



unions simply as fusion. The process of conjugation is here described 

 and illustrated as it occurs in Paramecium. 

 1/ At the time of conj ugation, Fig. 138,^, two individuals of Paramecium 

 come together with their oral surfaces in contact. They are held in this 

 position for a time because of the stickiness of the protoplasm on those 

 surfaces. While they continue to swim about internal changes in the 

 micronucleus and macronucleus of each individual take place. The 

 micronucleus of each Paramecium divides by a process of modified or 

 primitive mitosis, and then each half divides again. Thus each micro- 

 nucleus gives rise to 4 micronuclei (Fig. 138, C). Of these micronuclei, 

 three undergo degeneration, and the one remaining in each Paramecium 



B 



D E F 



Fig. 137. — Scheme of spore formation in gregarines. A, two individuals united in a 

 common cyst; B and C, gametes of similar size in process of formation; D, union of amoe- 

 boid gametes ; E and F, formation of sporozoites within the spores. {From Calkins's Protozoa. 

 Courtesy of Macmillan Co.) 



divides again into two parts, usually of unequal size {D and E) . The smaller 

 micronucleus of each individual now passes over into the other individual 

 (F) while the larger one is retained. The two pieces, one derived from 

 each individual, now fuse to make the fusion micronucleus (G). During 

 these stages of the process the macronucleus has been undergoing frag- 

 mentation and sooner or later its parts degenerate completely. The 

 parts, however, may linger for a considerable time and thus render diffi- 

 cult the interpretation of subsequent stages. Soon after the exchange 

 of micronuclei the individuals separate and the process of conjugation 

 itself is completed. Fusion of the micronuclei, however, initiates a 

 series of changes covering a long period. These processes in one of the 

 ex-conjugants are essentially as follows. The fusion micronucleus divides 



