130 . AN INTRODUCTION TO ZOOLOGY 



middle of which lie the chromosomes forming the equatorial plate. 

 The nucleus now elongates very markedly, and a group oi chromo- 

 somes pass towards each end. ^ Finally, the nuclear membrane 

 separates, and thus two daughter nuclei are formed which take up 

 positions near their corresponding macronuclei. Shortly after this 

 a circular furrow appears in the central region of the body, and 

 soon we have in the place of the one parent two daughter Para- 

 mcecia. Each grows until it attains the maximum size, and is then 

 ready to divide. 



The process of indirect nuclear division in the micronucleus is 

 more complex than in the nucleus of Amoeba proteus, and is more 

 advanced in that definite chromosomes, polar plates and a spindle 

 are produced in it, but it must be borne in mind that even here the 

 whole series of changes occurs within the nuclear membrane, a marked 

 difference from the indirect division that we meet in the higher 

 animals, as we shall see later. 



The normal method of reproduction, then, is this asexual one 

 by simple fission, a process taking from half an hour to two hours, 

 according to the temperature, and from the one individual a large 

 number are ultimately produced by repeated divisions. It has been 

 found that this process can go on for a long time, and then it gradually 

 slows down and ceases. Change of food will start it off afresh, and 

 an American observer was able to keep the process maintained for 

 nearly two years, during which 742 generations were passed through, 

 but after that no stimulus proved of any avail, and the animals 

 died off. Multiplication by fission can continue for some time, 

 but ultimately a limit is reached, and no change of diet will produce 

 the desired effect, and the Paramoecia, unless they are allowed to 

 conjugate, will die out.* The act of conjugation appears to bring 

 about a rejuvenescence, and after it, the two individuals concerned 

 serve as the starting points for new series of divisions. If Para- 

 mcecia be kept in an infusion it will be seen that every now and then 

 nearly all the animals come together in pairs, and what has been 

 termed an epidemic of conjugation sets in. Wha^ brings about this 

 impulse to conjugate is not known, but the changes that accompany 

 it have been studied fairly fully. The details vary in different 

 species, but the essential processes are the same, and we will now 

 consider them in P. caudatum. 



* More recent observations show that a race of Paramcecia can be kept 

 going almost indefinitely without conjugation, but in this case the individuals 

 undergo a series of regenerative nuclear changes within themselves. This 

 process of endomyxis, as it is termed, is somewhat similar to, and under some 

 conditions can undoubtedly take the place of conjugation. It is to be borne 

 in mind, however, that this occurs under laboratory conditions which are 

 practically inconceivable in the natural surroundings of the animal. 



