VI MINUTE STRUCTURE OF CELLS 6i 



leaves, and other portions of the plant may be shown to 

 consist of an aggregation of cells agreeing in essential 

 respects with the above description. 



We come therefore to a very remarkable result. The 

 higher animals and plants are built up — in part at least — of 

 elements which resemble in their essential features the 

 minute and lowly organisms studied in previous lessons. 

 Those elements are called by the general name of cells : 

 hence the higher organisms, whether plants or animals, are 

 multicellular or are to be considered as cell- aggregates, 

 while in the case of such beings as Amoeba, Hcematococ- 

 cus, Heteromita, or Euglena, the entire organism is a 

 single cell, or is unicellular. 



Note further that the cells of the higher animals and 

 plants, like entire unicellular organisms, may occur in either 

 the amoeboid (Fig. 8, a, bS c^) the ciliated (e), or the 

 encysted (f) condition, and that a plasmodial phase (b^) is 

 sometimes produced by the union of two or more amoeboid 

 cells. 



One of the most characteristic features in the unicellular 

 organisms described in the preceding lessons is the con- 

 stancy of the occurrence of binary fission as a mode of 

 multiplication. The analogy between these organisms and 

 the cells of the higher animals and plants becomes still 

 closer when we find that in the latter also simple fission is 

 the normal mode of multiplication, the increase in size of 

 growing parts being brought about by the continual division 

 of their constituent cells. 



The process of division in animal and vegetable cells 

 is frequently accompanied by certain very characteristic and 

 complicated changes in the nucleus to which we must now 



