THE LIFE OF THE SIMPLEST ANIMALS 11 
the case of the reproduction of Paramecium the beginnings 
of developmental changes during the course of the growth 
of the young. The young Ame@bde@ have only to add sub- 
stance to their bodies, to grow larger, in order to be exactly 
like their parent. 
The new Paramecia 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 Paramecia, 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 
Fie. 6.—Paramecium putorinum 
dividing. The two nuclei be- 
come very elongate before di- Fie. 7.—Paramecium caudatum ; two indi- 
viding.—After Birscu ii. viduals separating after conjugation. 
of one of the nuclei of the other member of the pair. In 
the meantime the second nucleus in each Paramecium 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 Paramecium, with its nuclei com- 
posed of parts of the nuclei from two distinct individuals, 
