206 



GENERAL PHYSIOLOGY 



That which comes to pass among the Protista in a single cell, 

 takes place in an aggregate of cells in the development of the 

 multicellular organism. In accordance with the above considerations 

 concerning reproduction, the development of the multicellular 

 organism from the unicellular egg can take place by continued cell- 

 division only. But in this process two factors play important rdhs : 

 first, the products of the division of the egg-cell do not separate 

 as in most Protista, but remain in connection with one another; 

 and, second, the products of division are not always alike, but by 

 unequal division two forms of cell, wholly different from each 

 other and from the mother-cell, can arise. In this manner is 



Fig. 85. — A. EudoHna elegans, B. Magosphara planula, two multicellular organisms consisting of 

 similar cells. (After Haeckel.) 



rendered possible the origin, not only of a multicellular organism, 

 but of such an organism with diiferentiatiou of various kinds of 

 tissues and organs. If the first factor alone were present, there 

 would result a cell-community consisting ' of many cells, all of 

 which, however, would be alike. Such organisms exist among 

 Protista (Fig. 85), and are regarded as cell-colonies that have 

 a republican constitution, i.e., in which every cell is exactly 

 like every other. These forms are the intermediate links be- 

 tween the really unicellular organisms and the animals or plants. 

 In the bodies of animals and plants, even the lowest, the cells 

 are not all alike, and this differentiation, through which alone 

 the development of a complex cell-community becomes possible, 

 depends upon the efficiency of the second factor, unequal cell- 

 division. Hence, cell-division, both equal and unequal, and 

 cohesion of the cells are the factors that bring about the develop- 

 ment of a differentiated cell-communitv. 



