THE MULTICELLULAR ORGANISM 



55 



species from which it has separated. Both the spore and the 

 zygote (fertilized egg) are set, one may say, to go through a 

 series of changes which transform an apparently simple cell 

 into an obviously complex multicellular plant or animal with 

 all the tissues and organs characteristic of the species. It is 

 important, at this point, to review the general method by 

 which the development of the adult is accomplished. 



Briefly, the modus operandi of development is cell division 

 accompanied by differentiation. The 

 spore (asexual) or the fertilized egg 

 (sexual) by a succession of cell divi- 

 sions, termed CLEAVAGE, passes 

 from the single-cell stage to a two- 

 cell stage and then, with more or 

 less regularity, to four-cell, eight- 

 cell, sixteen-cell stages, etc. If these 

 cells separated after each divi- 

 sion, the same general condition 

 would obtain here which has been 

 seen in the Protophyta and Proto- 

 zoa, where each organism is a com- 

 plete free-living cell. Or again, if 

 cleavage merely resulted in a group 

 of so many exactly similar cells, there would arise a colony of 

 unicellular individuals rather than a multicellular organism. 

 Such colonial forms are, in fact, numerous among the lower 

 plants and animals, and show nearly all grades of complexity 

 from simple associations of a few identical cells, as for example 

 in Spondylomorum, to groups of many thousands in which 

 some of the individuals are specialized for certain functions. 

 (Fig. 17.) Volvox affords an instructive example of the 

 latter condition. The majority of the cells, ten thousand or 

 more, which form the relatively large spherical colony are 



FIG. 17. A simple colony of 

 unicellular organisms (Spondylo- 

 morum) each of which carries on 

 all the functions of nutrition and 

 reproduction. Highly magnified. 

 (From Hegner, after Oltmanns.) 



