Reduction m the Protozoa 181 



so that finally the two half nuclei that mate have each 8 

 chromosomes. The original number, 16, is restored by the 

 mating. 



In Opalina intestinalis, a large infusorian parasitic in the 

 alimentary canal of amphibians, the number of chromosomes 

 during ordinary reproduction by fission is 8. In the spring 

 there appear small animals, which divide several times, then 

 encyst; when they come out of the cysts they mate, two 

 individuals completely uniting. These small individuals 

 before mating have but 4 chromosomes in place of 8. How 

 the reduction is brought about is not known (Metcalf, 1909). 



In the two common species of Paramecium, aurelia and 

 caudatum, the nuclear processes at mating appear to differ 

 considerably. They are best known in Paramecium cau- 

 datum, through the work of Calkins and Cull (1907). In 

 this animal the matter is greatly complicated by the fact 

 that a very large number of chromosomes is present (Figure 

 49). The number is so great that they cannot be counted, 

 but Calkins and Cull estimate them at about 165. 



In individuals beginning mating, the chromosomes appear 

 as double rods (Figure 49, A). Calkins and Cull suspect 

 that this is due to the pairing of two chromosomes, such as 

 we saw in Monocystis (Figure 46). At both the first and 

 second maturation divisions these double chromosomes split 

 lengthwise. One of these divisions therefore apparently 

 separates the paired chromosomes (reducing the number to 

 half in each resulting half nucleus ) ; the other divides each 

 chromosome lengthwise. The reduced number is apparently 

 therefore present in the micronuclei before the third division. 

 This third division takes place in a very different way from 

 the other two. The chromosomes, instead of being long 

 threads, fall into strings of granules ; and each of these 

 strings is broken transversely, at about its middle (G). 



