58 ANIMAL BIOLOGY 



by a succession of cell divisions, 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 division, the same general condi- 

 tion would occur here which has been seen in the Protozoa, where 

 each organism is a complete 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 of cells in which some of 

 the individuals are specialized for certain func- 

 tions. Volvox affords an instructive example 

 of the latter condition. The majority of the 

 cells, ten thousand or more, that form the 

 relatively large spherical colony are flagel- 

 lated, Euglena-like individuals, each of which 

 Fig. 29. — A simple lives a practically independent existence in 

 colony of unicellular organic union with its fellows. The chief con- 

 mwum m quaZ P n7tm) tribution of each of these cells to the economy 

 each of which carries of the whole results from the lashing of its 

 on all the functions of flagella, which helps to propel the colony 

 iuction C U mg rePr ° th rou &ri th e water. But, under certain condi- 

 tions, some of the cells become specialized 

 for reproduction, both asexual and sexual, and form new colonies 

 which sooner or later are set free. Thus we have a differentiation 

 of reproductive (germ) cells from body (somatic) cells, and a fore- 

 shadowing of that further specialization and physiological division 

 of labor between cells which is the most characteristic feature of the 

 higher organisms. (Figs. 29, 30, 154.) 



Indeed, the complex bodies of multicellular organisms are made 

 possible by cell differentiation and cell cooperation. All protoplasm 

 possesses, for instance, the primary attribute of contractility, but 

 in the muscle cells of animals we find the capacity for contraction 

 very greatly developed, and to a certain extent at the expense of 

 other powers. But this differentiation would be ineffective were 

 not innumerable similarly specialized cells grouped together — 



