46 MANUAL OF ANIMAL BIOLOGY 



the meeting and temporary fusion of two animals. This occurs in 

 such a way as to involve a considerable portion of the oral surfaces. 

 Thus a temporary protoplasmic bridge is formed between the ani- 

 mals. Coincident with this external fusion, as shown in figure 3, a 

 series of nuclear reorganization phenomena begins in each animal 

 which are of the same general character as in endomixis, and which 

 in Paramecium aurelia may be outlined as follows : 



(1) The two micronuclei in each animal become somewhat 

 enlarged, and two mitotic divisions occur, resulting in the forma- 

 tion of eight micronuclei in each animal. The macronucleus de- 

 generates and entirely disappears. (W. f. 170, A, B, C.) 



(2) Seven of the eight newly formed micronuclei in each animal 

 degenerate and the remaining one divides again to form two bodies, 

 termed gametic nuclei. (W. f. 170, D, E, F.) 



(3) One of the nuclei (stationary gametic nucleus) remains 

 in the animal where it was formed, but the other (migratory 

 gametic nucleus) moves across the protoplasmic bridge into the 

 other animal, where it permanently fuses (fertilization) with the 

 stationary gametic nucleus there present to form the synkaryon. 

 (W. f. 170, F, G, H.) 



(4) The exchange of nuclear material having been effected, the 

 two animals separate. In each exconjugant the new nuclear ap- 

 paratus is then built up by the division and differentiation of the 

 synkaryon. It divides twice in each exconjugant to form four 

 micronuclei, two of which remain as micronuclei and two enlarge 

 to form macronuclei. The two micronuclei in each exconjugant 

 then divide. This is followed by the division of each exconjugant 

 into two normal cells, each with a macronucleus and two micro- 

 nuclei. A period of reproduction by the regular process of cell 

 division now follows. (W. f. 170, /, J, K, L.) 



4. Adaptation 



The fact that Paramecium, because of its highly coordinated, 

 ciliary locomotor apparatus, is able to move in any direction with 

 considerable rapidity, makes it a valuable animal for studying its 

 reaction or 'motor response' to various kinds of external stimuli. 

 The latter may be caused by various changes in the environment 

 as a result of the action of chemical, electrical, photic, or 

 thermal phenomena. If, for example, a drop of some unfavorable 

 fluid is added to the culture medium containing Paramecia, when 



