Cell Division in Relation to Reproduction - 55 



Significance of Meiosis. Meiosis quite ob- 

 viously constitutes a mechanism whereby 

 haploid reproductive cells are formed from 

 diploid progenitors. During the two divi- 

 sions of the original cell, only one division 

 of the chromosomes occurs. Consequently, 

 meiosis inevitably produces a reduction in 

 the chromosome number. Meiosis and fer- 

 tilization together constitute a system by 

 which half the chromosomes of the new in- 

 dividual are derived from each of the par- 

 ents, without at the same time leading to a 

 progressive increase in the chromosome num- 

 ber of the species. In conjunction with fer- 

 tilization, meiosis provides a basis for sexual 

 reproduction, permitting the development of 

 biparental inheritance. Biparental inherit- 

 ance in turn greatly increases variability in 

 the inherited qualities within the species. In 

 species where the only method of cell multi- 

 plication is mitosis, the reproduction is 

 asexual; and since the chromosomes passed 

 on to the ensuing generations tend to be 

 identical with those of the parent, hereditary 

 variability in asexual forms remains at a 

 minimum. 



REPRODUCTION IN SIMPLER ORGANISMS 



Strictly speaking, the term "reproduction" 

 is restricted to processes whereby total new or- 

 ganisms are generated. Thus the mere mul- 

 tiplication of cells in a multicellular organ- 

 ism does not of itself constitute reproduction. 

 Only when the single cell is at the same time 

 a complete organism, as in the Protozoa and 

 other unicellular forms, does cell division 

 alone constitute a form of reproduction. 



The reproduction of an organism may be 

 sexual or asexual, depending upon whether 

 fertilization comes into play. All forms of 

 reproduction which involve fertilization are 

 designated as sexual, whereas all reproduc- 

 tive processes not involving fertilization are, 

 by this token, asexual. 



Asexual Forms of Reproduction. 



Binary Fission. Binary fission is the term em- 

 ployed when an organism divides directly 



into two approximately equal parts, each of 

 which then grows into an individual similar 

 to the departed parent. The commonest form 

 of binary fission results when a unicellular 

 organism undergoes mitosis (Fig. 3-7). How- 

 ever, there are some multicellular forms, like 

 certain flatworms, that reproduce by splitting 

 directly into two quite equal pieces (p. 395). 



Among protozoans, bacteria, and algae, 

 the two cells formed by binary fission usually 

 separate soon after division, swimming or 

 crawling away, each on its separate mission. 

 Consequently such species remain strictly 

 unicellular. But the cells of some nonmotile 

 forms adhere together throughout several 

 divisions. Thus they form incipient colonies 

 of loose and indefinite form. Such colonies 

 disintegrate readily with mechanical disturb- 

 ances such as currents in the surrounding 

 water. The formation of definite colonies de- 

 pends upon a more firmly established tend- 

 ency of cells to cling together through a 

 series of divisions; and in a true colony the 

 planes of division, and consequently the ar- 

 rangement of the cells, become fixed and 

 regular. In a linear colony such as Spirogyra, 

 for example, all cell divisions occur at right 

 angles to the length of the filament, and the 

 cells remain fastened together end to end 

 (Fig. 3-16). Such colonies cannot be dispersed 

 except by rather violent disturbances, or by 

 the death and disintegration of some of the 

 cells in the chain. Many cell aggregates, even 

 such definite colonies as Spirogyra, display 

 no real distinction between the reproduction 

 of the individual cells (cell division) and the 

 reproduction of the whole colony. Only after 

 further evolution — with the development of 

 special cells that assume the reproductive 

 function of the colony or organism — does this 

 distinction become plain. 



Although binary fission is probably the 

 oldest and most primitive kind of reproduc- 

 tion, it is not restricted entirely to unicellular 

 forms. A few multicellular animals such as 

 Hydra (Fig. 3-12) and Planaria (Fig. 16-7) 

 can also reproduce by fission. When a multi- 

 cellular organism splits into two quite equal 



