110 Life and Death, Heredity and Evolution 



As you recall, Paramecium multiplies for many genera- 

 tions in single lines, the offspring having but one parent. 

 Then mating occurs ; the animals place themselves with their 

 oral sides together and become partly united (Figure 6). 

 The surfaces of the two adhere, and at a certain spot the 

 interior protoplasm of the two comes into actual union. The 

 main things that then occur are those shown in Figure 8 

 (page 26), and in Figure 30. The old active nucleus (mac- 

 ronucleus) breaks in pieces and is gradually absorbed, like 

 so much food; it disappears completely. This really re- 

 quires a long time, so that fragments of it are still found 

 in later stages, but as this has no importance, these frag- 

 ments are omitted from the figures in order not to confuse 

 them. The single small reserve nucleus (micronucleus) 

 divides twice, into four (Figure 30, A, B, C), and three of 

 these are absorbed and disappear (C) like the macronu- 

 cleus. Then the remaining (fourth) one divides into two 

 parts (see Figure 40). Of these two parts, one lies a little 

 nearer the surface of union of the two conjugants, and is a 

 little smaller than the other. This one begins to move 

 toward the opposite conjugant; it is therefore commonly 

 spoken of as the "migratory" half nucleus. In the other 

 conjugant the same thing happens, so that the two migra- 

 tory half nuclei meet and pass each other at the boundary 

 between the two conjugating individuals (Figure 40, B, C, 

 D). Each of the two stationary halves remains in place 

 till the migratory half nucleus from the opposite individual 

 reaches it; then the migratory and stationary half nuclei 

 unite (Figure 30, E; Figure 40, E, F). 



Thus the general upshot of the process is that the two 

 mating animals exchange halves of their micronuclei, and 

 at the end each has a micronucleus composed of substance 

 partly from one mate, partly from the other. This is evi- 

 dently the central point in the conjugation. 



