356 ANIMAL BIOLOGY. [Part II. 



rudimentary form of digestion. This process, and the energy 

 involved in the constant change of form, are the indications of 

 hidden metabolic processes which go on within the cell ; and 

 there can be no doubt that these metabolic changes involve 

 processes of respiration and the excretion of waste products. 

 Whether there is any part of the cell which merely forms a 

 framework for the support of the rest it is difficult to say. 

 Finally the simple cell is reproductive. 



We have already seen that the metazoan differs from the pro- 

 tozoan in being a cell-aggregate. We may now note that as we 

 advance towards the higher metazoa we find a more and more 

 perfect division of labour among the cells special cells taking 

 upon themselves the various functions which, in the amoeba, are 

 performed in a rudimentary fashion by the single cell. Or we 

 may say to put the matter in another way that in the higher 

 metazoa there is such differentiation among the cells that special 

 cells or groups of cells are enabled to cultivate, almost exclusively, 

 some one or other of the protoplasmic faculties. Some (nerve) 

 become eminently sensitive ; others (muscle) eminently contrac- 

 tile ; yet others (liver) eminently metabolic, secretory (salivary), 

 or excretory. All or most are capable of reproducing by fission 

 other cells like themselves ; but to certain cells are intrusted 

 the all-important function of continuing the race by reproducing 

 complex individuals inheriting the peculiarities of the parent 

 organism. So important are these cells that it has been sug- 

 gested that all other tissues and organs exist that the ovary may 

 produce an ovum which shall be duly fertilised, 



Be this as it may, we see that the higher metazoan individual 

 is a cell-aggregate in which there has gone on a large amount of 

 differentiation and division of labour among the constituent cells. 

 But here we must further note that accompanying the differen- 

 tiation there is also integration. It would be of no use for the 

 cells to become divergent in structure and function if, accom- 

 panying this essential process, there were not a process, equally 

 essential, by which the cells are knit together into the tissues 

 and organs of one individual body. For efficient advance in the 

 life-scale integration and differentiation must go hand in hand. 



