162 



PRINCIPLES OF ANIMAL BIOLOGY 



because in Volvox and Pleodorina the reduction in the number of chro- 

 mosomes occurs, not just before the cells are ready for reproduction, but 

 a long time earlier. Indeed, all the cells of these organisms have the half 

 number of chromosomes; only the zygote from which they spring has 

 the full number. In Pandorina morum (Fig. L31) there is a curious com- 

 bination of isogamy and anisogamy; it has reproductive cells of two sizes, 

 and union may occur between two small ones or between a large and a 

 small one, but not between two large ones. 



In the foregoing examples, union of gametes is a fusion of whole 

 cells. In some of the ciliated protozoa, however, it is only the nuclei 

 of the cells which fuse. In the species in which this occurs, there are 



Fig. 1.31. — Reproduction in Pandorina morum Borg. A, vegetative colony; B, asexual 

 reproduetion; C, gametes (y) ; D-E, union of gametes to form zygote {z) ; F-H, development 

 of zygote. {From West after Pringsheim.) 



two nuclei in each individual, a large one called a macronucleus and a 

 small one, or micronucleus. Only the micronuclei are involved in the 

 union, and it is justifiable to regard these nuclei, rather than the whole 

 cells, as the gametes. To effect this union, the cells must come together 

 temporarily and make an exchange of nuclei. Temporary union of 

 two protozoan individuals for exchange of nuclei is called conjugation. 

 Since the process is rather complicated, it is best illustrated by a specific 

 example, for which Paramecium is selected. 



At the time of conjugation (Fig. 132.1) two individual paramecia 

 come together with their oral surfaces in contact. They are held in 

 this position for a time because of the sti(;kiness of the protoplasm 

 on those siu'faces. While they continue to swim about, internal changes 

 in the micronucleus and macronucleus of each individual take place. 

 The micronucleus of each Paramecium divides by mitosis {B, C, D), 



