METABOLISM, IRRITABILITY, AND REPRODUCTION 193 



because metazoan cells are limited by their specialization. Even 

 the cells of sense-organs, which are par excellence the cells of irri- 

 tability, have limited powers in the metazoa, for they respond only 

 to particular forms of stimulation — the auditory cells only to 

 sound waves, the retinal cells only to light, the cells of the taste- 

 buds to certain chemicals. Nerve cells may respond more widely, 

 as when one stimulates a nerve by heat, by chemicals, or by elec- 

 tricity; but when a nerve impulse reaches other nerve cells in the 

 central system, the effect may be similar, because the cells seem 

 able to respond only ii- a hmited manner. While the responses 

 of the metazoan body as a whole may be far more complex than 

 those of any protozoan, it is, nevertheless, difficult to make out 

 for the majority of cells in the metazoan even as wide a range cf 

 response to stimulation as occurs in the more active protozoa, for 

 the reason that the cells of metazoa are speciahzed for particular 

 functions while the protozoan cell is speciahzed for all the functions 

 of an individual. In any event, it is clear that irritabihty is the 

 same kind of a process in both protozoa and metazoa. 



The exact manner in which the reproductive process may be 

 compared in protozoa and metazoa is described in a subsequent 

 chapter. In this connection, however, it may be repeated that 

 conjugation, as it occurs in a majority of the protozoa, by a per- 

 manent fusion of two cells to form one, is comparable with fer- 

 tihzation, or the union of egg with sperm, in the metazoa. From 

 a zygote arising in this manner, many independent protozoan 

 cells are formed by division; while in metazoa the zygote produces, 

 by cell division and differentiation, a many-celled body. Hence, 

 there is a remarkable parallehsm between the cellular cycles even 

 in the extremes of animal life {cf. Fig. 110, p. 215). Reproduction, 

 hke metabolism and irritabihty, is a similar process whether in the 

 highest or the lowest animals. The essential nucure of the cell 

 and its protoplasm and the universality of the distinguishing 

 capacities cf protoplasm become increasingly apparent as we 

 proceed. 



The foregoing comparison of ceU with cell in protozoa and meta- 

 zoa perhaps does injustice to the protozoan as an individual animal. 

 Protozoa are " cells," but they are also " individuals." As such 

 they may be compared with individuals composed of many cells. 

 In extreme cases (Fig. 102) the single-cehed organism may present 

 features that parallel structures in higher animals and thus may 



