ASSIMILATION. 19 



work does and so at last equals the reception and construc- 

 tion. The second limitation to indefinite growth is con- 

 nected with the power of the cell to give rise to new cells 

 like itself. Under certain circumstances, which are not 

 well known, the cell may become narrowed (Fig. 6) at one 

 zone; the constriction deepens until the parts on each side 

 of it are merely united by a narrow band which finally 

 gives way and two cells are formed, each like the parent 

 but for its smaller size; or the cell may divide into two or 

 more by flat surfaces of separation, or in 

 a way intermediate between this mode 

 and the last (Fig. 7). In some cases new 

 cells form in the interior of the old and 

 are then set free from it. The new cells 

 produced in these ways grow as the origi- 



J . c i,- i FIG. 7. Another 



nal cell did, and may in turn multiply in mode of ceil division. 



TT , , T A rounded cell elon- 



the same manner. Very commonly tne gates in one diame- 

 nucleus divides before the rest of the nfrrow^nl'nSaS 

 cell, and its parts then form the nuclei of 2&i tf $JS 



+lia nmv ppllo parts; the nucleus di- 



tne new ceils. vides at the same 



Assimilation: Reproduction. These time - 

 two powers, that of working up into their 

 own substance materials derived from outside, known as 

 assimilation, and that of, in one way or another, giving rise 

 to new beings like themselves, known as reproduction, are 

 possessed by all kinds of living beings, w*hether animals or 

 plants. There is, however, this important difference be- 

 tween the two: the power of assimilation is necessary for 

 the maintenance of each individual cell, plant, or animal, 

 since the already existing living material is constantly 

 breaking down and being removed as long as life lasts, and 

 the loss must be made good if anyone is to continue its ex- 

 istence. The power of reproduction, on the other hand, is 

 necessary only for the continuance of the kind or race, and 

 need be, and often is, possessed only by some of the indi- 

 viduals composing it. Working bees, for example, cannot 

 reproduce their kind, that duty being left to the queen-bee 

 and the drones of each hive. 



The breaking down of already existing chemical com- 



