THE LIFE OF THE SIMPLEST ANIMALS 



11 



the case of the reproduction of Paramcecium the beginnings 

 of developmental changes during the course of the growth 

 of the young. The young Amcebce have only to add sub- 

 stance to their bodies, to grow larger, in order to be exactly 

 like their parent. 



The new Paramcecia attain full size and then divide, 

 each into two. And so on for many generations. But it 

 has been discovered that this simplest kind of reproduction 

 can not go on indefinitely. After a number of generations 

 the Paramcecia, instead of simply dividing in two, come 



together in pairs, and a part of 

 one of the nuclei of each mem- 

 ber of a pair passes into the 

 body of and fuses with a part 



FIG. 6. Paramcecium putorinum 

 dividing. The two nuclei be- 

 come very elongate before di- 

 viding. After BiJTscHLi. 



PIG. 7. Paramcecium caudatum ,* two indi- 

 viduals separating after conjugation. 



of one of the nuclei of the other member of the pair. In 

 the meantime the second nucleus in each Paramcecium has 

 broken up into small pieces and disappeared. The new 

 nucleus composed of parts of the nuclei from two animals 

 divides, giving each animal two nuclei just as it had before 

 this extraordinary process, which is called conjugation, 

 began (Fig. 7). Each Paramcecium, with its nuclei com- 

 posed of parts of the nuclei from two distinct individuals, 



