Cell Division in Relation fo Reproduction - 61 



CHLAMYDOMONAS 



PANDORINA 



ZYGOTE 



Fig. 3-19. Sexual reproduction of several Volvocaceae. 



stage, constitutes the life cycle in each species. 

 In some species, the life cycle is very simple, 

 but in others, as we shall see, it is quite com- 

 plex. 



Some amoebae and other unicellular or- 

 ganisms that reproduce solely by binary fis- 

 sion display the very simplest type of life 

 cycle. A young amoeba feeds, grows, and 

 then divides, which returns the species di- 

 rectly to the starting point. Such a cycle is 

 devoid of any sexual stages, and consequently 

 the terms diploid and haploid have no real 

 meaning. Without fertilization no diploid 

 stage is ever formed, but by convention the 

 cells in such species are regarded as haploid 

 (Fig. 3-20). 



Another type of life cycle, which charac- 

 terizes many green algae, is exhibited by 

 Spirogyra. During the major part of the life 

 cycle, while the colony is multiplying asexu- 

 ally by mitosis, the cells remain haploid. 

 The diploid stage starts at the time of fer- 

 tilization and persists only until meiosis oc- 

 curs. This is in the spring, just before the 



zygospore germinates to form a new filament, 

 which again is haploid (Fig. 3-20). 



The opposite type of cycle is found in 

 almost all multicellular animals, including 

 man, and in a few primitive plants. In man, 

 the diploid stage begins when the egg is fer- 

 tilized, and persists in all the body cells, 

 which arise from the fertilized egg by mitosis. 

 Meiosis occurs very late in the cycle, when 

 the eggs and sperm are being produced in 

 the sex organs. This type of cycle is shown in 

 Figure 3-20. 



The final type of cycle, which is interme- 

 diate between the preceding two, is found in 

 all higher plants (Chap. 12), and in a few 

 multicellular algae. In this type of cycle (Fig. 

 3-20), there is a regular alternation of hap- 

 loid and diploid generations. The fertilized 

 egg always gives rise to a diploid plant 

 body that reproduces asexually— by forming 

 spores. This diploid asexual generation in 

 higher plants is therefore called the sporo- 

 phyte generation. The spores in all higher 

 plants are produced by meiosis in groups of 



