PROTOZOA 



85 



trunk of Carchesium, while even stalk fibers are absent in 

 Epistylis. 



Conjugation 



Another characteristic of the infusoria is the peculiar method 

 of fertilization by conjugation. Two individuals come together 

 and unite at some limited portion, usually in the region about 

 the mouth, thus forming a protoplasmic bridge between the two 

 organisms (Fig. 52). They remain thus fused for approximately 



Fig. 52— CONJUGATION STAGE OF UROLEPTUS 

 MOBILIS 



The two individuals remain fused in the manner shown for 

 24 hours when they separate. During this time a com- 

 plicated series of changes occur in the micronuclei 



From a photomicrograph by the author 

 Magnification, 600 



twenty-four hours, when they separate. In the meantime, how- 

 ever, profound changes occur in the cell body of each conjugant. 

 Three consecutive times the micronucleus in each enlarges and 

 divides, forming eight nuclei. These are the so-called meiotic* 

 divisions, the result of which is the reduction to one-half of the 

 normal number of chromosomes. Maturation'*' of germ cells, 

 involving the reduction divisions, is characteristic of all higher 

 animals and plants. * It is a remarkable evidence of the unity 

 underlying all life to find similar processes in the simplest one- 

 celled animals. After the third division one of the resulting 

 nuclei in each individual moves through the protoplasmic con- 

 necting bridge into the other individual where it fuses with a 



* Meiotic, from the Greek, meaning to make smaller. 



t Maturation is the final series of stages in the formation of the egg or the 

 sperm, by which its nucleus is prepared for union. 



t See illustration on page 49 of "Heredity and Variation" in this Scries. 



