UNICELLULAR ORGANISMS 65 



this disappearance of the power of multiplication and the nat- 

 ural disappearance of the race is prevented by the occurrence 

 of another process known as conjugation (Lat. con = together 

 + jugare = to join). 



Conjugation. Two individual Parameda come together and 

 place themselves side by side, adhering to each other as shown in 

 Fig. 24 a. They do not actually fuse together, but remain at- 

 tached. The micronucleus in each undergoes a series of changes 

 which results in its dividing into several parts, three of which 

 degenerate and disappear; c. Soon the fourth divides again into 

 two, one of which is slightly larger than the other; d. The 

 smaller part resulting from this last division passes over into 

 the other of the two conjugating individuals, the two animals 

 thus exchanging nuclear matter with each other, as shown by 

 the arrows in d. This small piece of the micronucleus, thus 

 exchanged by each individual, unites in each case with the larger 

 piece of the nucleus remaining in the other individual, and the 

 two combine to form a new nucleus, a fusion nucleus, shown 

 at /. The animals now separate, each of them carrying off in 

 itself a bit of the micronucleus from the other individual. The 

 old macronucleus next disintegrates and disappears (/), and the 

 fusion nucleus divides into eight parts (g), three of which soon 

 degenerate. One of the five that are left remains as a micro- 

 nucleus, while the other four become macronuclei, at h. At this 

 stage of the process each Paramecium has one micronucleus and 

 four macronuclei. Next the micronucleus divides into two, 

 and the entire animal divides at once into two separate parts, 

 giving one-half of the micronucleus to each part. This gives two 

 individuals, each with a micronucleus and two macronuclei; i to 

 k. The process is again repeated, the micronucleus and the 

 whole animal, except the macronuclei, dividing; the result is two 

 more individuals, each containing one micronucleus and one 

 macronucleus; I to m. This brings the animal back to its original 

 condition, and now the ordinary process of fission begins and 

 may go on again indefinitely, both micro- and macro-nuclei 



