DIVERSE MODES OF REPRODUCTION 



29 



2. Diverse Modes of Reproduction 



In the preceding paragraph we have given prominence to 

 what is true of the great majority of living creatures, that a 

 new life begins as a fertilised egg-cell. It is necessary, however, 

 to refer to the other ways in which a new organism may arise, 

 for some of them help us to understand what the hereditary 

 relation means. The following scheme will probably serve to 

 recall the familiar facts : 



Multiplication 



, f By division into two. 

 '' ar \ By budding, a modified form of division. 

 \ By sporulation, or division into many unit 



In unicellular 



organisms. ^ By sporulation, or division into many units. 



The reproduction may be wholly asexual : (i) in the sense 

 that there is nothing corresponding to fertilisation or amphimixis ; 

 and (2) in the sense that there are no special germ-cells. But in 

 many unicellular organisms there are elaborate processes of am- 

 phimixis, and in colonial forms, like Volvox, there is a definite 

 beginning of egg-cells and sperm-cells. Among the parasitic 

 Sporozoa or Gregarines in the wide sense there is also a close 

 approximation to the mode of sexual reproduction seen in most 

 multicellular organisms. No hard-and-fast line can be drawn. 



C I. Without special germ-cells e.g. by division 

 In multicellular J of the body, by giving off buds (and as the 



organisms. \ result of artificial cutting). 



1. II. With special germ-cells : 

 (a) Eggs from one parent are fertilised by sperms from another 



parent heterogamy, the commonest mode ; 

 () Eggs from one parent are fertilised by sperms from the same 



(hermaphrodite) parent autogamy, a very rare mode. 

 (0 Eggs may develop without fertilisation parthenogenesis. 



[A multicellular organism may also multiply by spore- 

 cells specialised germ-cells, yet hardly equivalent to eggs 

 which do not require fertilisation.] * 



If we lay emphasis on the presence or absence of special reproductive elements, the classifi- 

 cation of the modes of multiplication would read as follows : 



I. Without special repro- / Division, budding, etc., in most unicellulars. 



ductive elements. \ Division, budding, etc., in some multicellulars. 



( More or less distinct specialisation of reproductive elements in 

 II. With special repro-J some unicellulars. 



ductive elements, j Specialised ova and spermatozoa in most multicellulars. 

 I. Formation of spore-cells in some multicellulars. 



If we lay emphasis on the occurrence or non-occurrence of amphimixis ( 

 fication of the modes of reproduction would read as follows : 



I. Without any form of_ 

 amphimixis. 



II. With some form 

 amphimixis. 



fertilisation) the classi- 



Without special reproductive-cells: (a) division, budding, etc., 



in many unicellulars ; and (6) division, budding, etc., in 



some multicellulars. 

 With special reproductive-cells : (a) formation of spores in some 



multicellulars ; (A) parthenogenetic ova. 

 Without specialised reproductive elements, amphimixis occurs 



in most unicellulars. 

 With specialised reproductive elements, amphimixis occurs in 



a few unicellulars and in most multicellulars. 



