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 : 



/ In unicellular 

 organisms. 



Multiplication 



By division into two. 



By budding, a modified form of division. 



By sporulation, or division into many units. 

 The reproduction may be wholly asexual : (1) 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. 



In multicelhtlai 

 organisms. 



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

 of the body, by giving off buds (and as the 

 result of artificial cutting). 

 II. With special germ-cells : 

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



parent — heterogamy, the commonest mode ; 

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



^hermaphrodite) parent — autogamy, a very rare mode. 

 (c) 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 

 some unicellulars. 

 Specialised ova and spermatozoa in most multicellulars. 

 Formation of spore-cells in some multicellulars. 



If we lay emphasis on the occurrence or non-occurrence of amphimixis (= fertilisation) the classi- 

 fication of the modes of reproduction would read as follows : 



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

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

 some multicellulars. 

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



L multicellulars ; (b) 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. 



I. Without any form ofj 

 amphimixis. 



II. With some form 

 amphimixis. 



