266 BIOLOGY 



DISTRIBUTION OF SEXUAL REPRODUCTION 



Sexual reproduction, using this term to include conjugation, 

 is very widely distributed among organisms and, indeed, is 

 possibly coextensive with life. It is true that there are many 

 forms of unicellular animals and plants in which it has never 

 been shown to occur; but in many cases this is due to incom- 

 plete knowledge. With increasing knowledge, more and more 

 of the unicellular organisms are known to go through the proc- 

 ess of cell union under some conditions. Even some of the 

 longest known and best studied organisms (Amoeba) have 

 been recently shown to undergo conjugation. Among some 

 of the unicellular forms, too, there occurs a true sexual union. 

 In the malarial organism, for example, there is at one stage 

 in the life history a union of two unlike cells, which are regarded 

 as male and female, and a similar differentiation of uniting 

 bodies has been found in many other single-celled organisms. 

 The continued discovery of new examples of sexual union or 

 conjugation, among the lower organisms previously supposed 

 not to have this power, has led to a belief that a union of cells 

 in reproduction may be a universal characteristic of all life, 

 even though there are still many of the lower animals and 

 plants in which it has not been found. This conclusion is 

 as yet by no means proved and may not turn out to be strictly 

 true. In all groups of animals above the unicellular types, 

 sexual reproduction, by the union of true male and female 

 cells, is universal, and in the higher groups it is the only method 

 of multiplication known to occur. 



REPRODUCTIVE BODIES OR REPRODUCTIVE CELLS 



This term refers to the parts which are separated from the 

 bodies of animals or plants, and capable of growing into new 

 individuals. Sometimes they are multicellular fragments, like 

 the buds of Hydra or the gemmce of a plant; but in such cases 

 the term reproductive body is not usually applied to them. 

 In the large majority of cases the bodies formed for reproductive 



