156 CONJUGATION, MATURATION, AND FERTILIZATION 



of the common parental cyst, the gametes from one cell ultimately 

 meeting and fusing with those of the other (Fig. 75, p. 181). 



If, in cases like the preceding, the coupled cells should separate, the 

 process would be analogous to that characteristic of the infusoria, and 

 such processes may give a clue to the explanation of the highly enig- 

 matical processes in the latter group, where copulation, including 

 mutual fertilization, takes the place of gamete formation. A typical 

 example of this type of isogamy is that of Paramecium aurelia (cauda- 

 tum), which may be briefly outlined as follows: 



A culture of Paramecium aurelia can be easily prepared in the 

 laboratory by seeding a hay infusion with a dozen or more cells from 

 pond water. After some weeks they will have accumulated in great 

 numbers, and quantities of conjugating forms may be obtained by 

 removing them to watch glasses. Pearl ('07) has shown biometrically 

 that the "conjugating population" consists of individuals of measur- 

 ably smaller size than those of the usual pond water. There is also a 

 difference in the physical and chemical make-up of the cells, by which 

 the protoplasm becomes much more sticky, so that two individuals 

 upon meeting frequently fuse at any point, but this extremely miscible 

 condition is probably evidence of physiological weakness indicative 

 of old age, and represents an excess of the conditions under which 

 conjugation is possible. 



The union of the two paramecium cells is apparently the signal for the 

 beginning of the maturation processes of the nucleus (Fig. 68). In 

 many egg cells of metazoa, and in all spermatic cells, these processes 

 precede union, showing that they are more generally phenomena of the 

 ripening or maturity of a cell than phenomena induced by cell union, as 

 in paramecium. At the outset the two organisms are more loosely 

 attached, so that forceful ejection from a pipette is sufficient to separate 

 them. After twelve hours' union, however, the attachment is so firm 

 that no amount of force will break them apart without killing one or 

 both. Such forcibly separated conjugants are by no means without 

 vitality, five out of twelve which were followed in cultures continuing 

 to live and divide, one being followed through more than 158 genera- 

 tions before it was abandoned. 



The normal course of conjugation requires from eighteen to thirty 

 hours, according to the temperature, and during the process the 

 micronucleus of each cell divides twice; one of the four cells in each 

 case then divides again into dimorphic nuclei. One of these nuclei is 

 smaller than the other and acts as a spermatic or wandering nucleus, 

 while the other remains in the parent cell. Each cell receives a wan- 

 dering smaller micronucleus from the other organism; this fuses with 

 the larger micronucleus to form the fertilization nucleus of the new 

 individuals. Each fertilization nucleus then divides three times in 

 quick succession, and eight micronuclei are formed. Four of these 



