Reproduction in Multicellular Animals - 395 



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Fig. 21-18. Colony formation due to fission, in a flatworm (Stenostomum 

 grande). The eyespots of the members of the colony are shown as small circles. 

 Semidiagrammatic. (After Child.) 



asexual method of reproduction is called 

 budding. Among multicellular organisms the 

 equal or unequal fragments that generate 

 such new individuals consist of several or 

 many cells. 



Some coelenterates, flatworms, and anne- 

 lids regularly multiply by fission. Among the 

 flatworms the division is always transverse, 

 forming two elongate pieces, which separate 

 and regenerate the missing organs. In some 

 worms, however, the separation is delayed 

 until several other fissions begin to cut across 

 the serially arranged pieces (Fig. 21-18). Such 

 a process of fission gives rise to a chainlike 

 colony, which persists until the individuals 

 separate. From an evolutionary viewpoint, 

 this type of fission may be important, because 

 it may provide a clue as to how segmented 

 animals may have arisen. In fact it is con- 

 sidered possible that the segmented type of 

 body, which occurs in many higher animals, 

 arose from a permanent association of such 

 colonial individuals. 



In some animals the young embryo under- 

 goes fission, and this process of polyembryony 

 gives rise to two or more embryos derived 

 from a single egg. Certain parasitic "bees" 

 and "wasps" produce hundreds of offspring 

 from each egg; and one mammal, the nine- 

 handed armadillo, regularly produces four 

 young, by two fissions of each early embryo. 

 Likewise polyembryony sometimes occurs in 

 man and other mammals. In man, the result 

 is usually identical twins — although multiple 

 fissions, resulting in identical triplets, quad- 

 ruplets, and even quintuplets, have been re- 

 corded. These offspring bear a very close 

 resemblance one to another, because they all 

 arise from the same zygote, and all receive 

 an identical set of genes and chromosomes. 

 Ordinary fraternal twins, unlike identical 

 twins, arise from separate zygotes, and conse- 



Fig. 21-19. Considerable structural and functional 

 differentiation may occur between members of a colony 

 —as is shown by Obelia, one of the coelenterates. 

 The upper (four) members are nutritive zooids, which 

 ingest and digest food for the colony as a whole, 

 whereas the lower members are reproductive zooids, 

 which are responsible for reproduction. 



