172 



UNICELLULAR ANIMALS. 



taining an equal amount of material from each individual. This micro- 

 nucleus now divides twice and gives rise to four bodies, two of which be- 

 come macronuclei and two micronuclei. Fission next occurs, and is there- 

 after continued in the usual manner. 



This is a process clearly analogous to the union of the germ- 

 cells of higher animals. It cannot, however, be called gamo- 

 genesis or even reproduction ; it is only comparable with one of 

 the elements of gamogenesis. In the metazoon a fusion of two 



Fig. 89.— Group of Vorticellce, in various attitudes, attached to the surface of a 



water-plant. 



cells (fertilization) is followed by a long series of cell-divisions 

 (cleavage of the ovum), the resulting cells being associated to 

 form one new individual. In the Infusoria temporary fusion 

 (conjugation) is likewise followed by a series of cell-divisions, 

 but the cells become entirely separate, each being an individual. 

 Vorticella agrees with Paj^mnmcium in general structure, but 

 differs in many interesting details, most of which are the expres- 



