Reduction in the Protozoa 177 



with four chromosomes, unite, forming a new nucleus with 

 eight chromosomes. In this entire process of reducing the 

 number to four and then, by mating, restoring it to eight, 

 of course many different combinations of the chromosomes 

 may arise in the different resulting individuals, in the way 

 already set forth. The number of possible diverse com- 

 binations in this case with four pairs of chromosomes, is, 

 as we have seen, 81. 



Reduction is better known in the ciliate infusoria than 

 in any other group of Protozoa. To understand what hap- 

 pens, one must recall the fact that at the beginning of mat- 

 ing there are three successive divisions of the micronucleus, 

 the third one producing the migratory and stationary half 

 nuclei. These are indicated in Figure 35. These three 

 divisions are commonly spoken of as the first, second and 

 third maturation divisions; we shall employ these designa- 

 tions. 



In the infusorian Didinium nasutum (Figure 47), accord- 

 ing to Prandtl (1906), there are 16 minute chromosomes 

 (A). In the first of the three maturation divisions each of 

 these 16 chromosomes divides into 2, so that the resulting 

 two micronuclei still have 16 chromosomes (Figure 47, B). 

 But in the second division, the 16 chromosomes merely sep- 

 arate into two groups of 8, one group going to each of the 

 two resulting micronuclei (C, D, E, F). In the third 

 division (G, H, I) each of the 8 chromosomes present divides 

 into two, so that each of the two half nuclei now has 8 

 chromosomes. Now the migratory half nucleus from one 

 mate passes over and unites with the stationary half nucleus 

 of the other (Figure 47, J, K, L), so that the resulting 

 complete nucleus now has 16 chromosomes. In the later 

 divisions of this complete nucleus, each of the 16 chromo- 

 somes divides (M), so that all the nuclei of later generationa 



