THE MULTICELLULAR ANIMAL 



59 



marshalled to perform a certain function. The power of a muscle 

 results from the combined action of its component muscle cells. 

 Differentiation without cooperation between cells we have seen 

 in unicellular organisms. Paramecium has highly specialized 

 parts ; other Protozoa have still more — perhaps the limit of possi- 



Fig. 30. — Volvox globator, a large colony of flagellated unicellular organ- 

 isms in which the various cells have become organically connected, and certain 

 cells specialized for reproduction. (Highly magnified.) A, mature colony 

 (highly magnified) showing sperm, d\ and eggs, 9, in various stages of de- 

 velopment; B, four cells (more highly magnified) showing the connections 

 between three 'somatic' cells, and the early differentiation of a reproductive 

 cell, rp. cv, contractile vacuole; st, 'eyespot' or stigma. (From Kolliker.) 



bilities of the single cell. But the multicellular body solves the 

 difficulty by removing the limit — by assigning special functions 

 to groups of cells rather than to parts of one cell. 



Thus cell division (cleavage) involving growth, differentiation, 

 and its attendant physiological division of labor is the keynote 

 of development in the higher animals and plants. Among animals, 

 the cells which arise from the cleaving egg (zygote) usually become 



