163 THE ANIMAL KINGDOM 



Successive mitosis 



■^ ^ ^ 





Ferlilixeilion Encystm^nt MeiosiS 





V 



^' 







Succi^ssive. mitosis in haploid stage 



Figure 8.16. Sexual cycle in the Phytomonadina. Ordinary individuals are haploid 

 (outsides of diagram) and reproduce asexually. Under certain conditions they unite in 

 pairs (left) to form a zygote that encysts. Within the cyst meiosis occurs, so that when 

 the individuals emerge (right) they are haploid again. 



Pairing 1st meiotiC Sndmeiotic Haploid Mutual Nuclear fusion 



division. division mitosis f ertilixalion. 



Figure 8.17. Sexual cycle in Paramecium. Two individuals with diploid micro- 

 nuclei unite in conjugation (left). After meiosis (second and third figures) three of the 

 products degenerate and the fourth divides by mitosis (fourth figure). Mutual fertiliza- 

 tion is followed by fusion of the haploid nuclei to form a new diploid nucleus (last 

 figure). The old macronuclei disappear. The new diploid nuclei divide several times by 

 mitosis, and eventually establish both the new macronuclei and the new micronuclei. 



but immediately undergoes meiosis to produce active individuals (Fig. 

 8.16). These may divide mitotically to produce large populations of 

 individuals. In the simplest case, at the time of sexual reproduction 

 two individuals of opposite sex fuse together to form the zygote. In 

 some species, especially in colonial forms like I'olvox, individual cells 

 undergo metamorphosis before functioning as gametes. In one sex the 

 metamorphosing cell becomes large and egglike, while in the other sex 

 the metamorphosing cell divides rapidly to produce a number of small 

 spermlike gametes. In these species the sexes can be designated as male 

 and female. Only the haploid stages are sexual, however; the zygotes 

 are indeterminate as to sex, and in a given species the meiotic process 

 is identical for all zygotes whether the ultimate gametes be eggs or 

 sperm. 



