2 THE PROTOZOA 



tively constitute a higher animal. The Protozoa are, in short, com- 

 plete, but unicellular organisms, and are to be regarded as the most 

 generalized of single cells. 



Considered as a complete animal, the protozoon cell at once arouses 

 the inquiry as to the nature of the organs by means of which the vital 

 functions are carried on. Lending themselves readily to the experi- 

 mental method of investigation, the Protozoa have already contributed 

 not a little to knowledge of the localization of function in the cell. 

 The importance of the nucleus in the economy of cell-life, which 

 Barry and Goodsir early pointed out in animal tissues, has been fully 

 confirmed by the researches of Gruber, Balbiani, Hofer, Verworn, and 

 others upon the Protozoa. From the structural point of view, the 

 protozoon nucleus with its accompanying structures must ultimately 

 throw considerable light on the vexed questions connected with the 

 finer structures of metazoan cells. As the sequel will show, consider- 

 able advance has already been made in this direction through the 

 efforts of Biitschli, Schaudinn, Balbiani, R. Hertwig, and many others. 

 Here, the generalized structures, especially those elements concerned 

 in cell-division, although difficult of analysis, must, when more 

 thoroughly studied, aid the interpretation of the more specialized 

 structures in Metazoa which are now involved in some of the most 

 deeply-lying problems of biology. 



Physiology likewise has been and is still to be greatly enriched 

 by the study of unicellular animals. Bichat's theory of tissues, pro- 

 pounded at the very outset of the last century (1801), formed the 

 basis of Virchow's development of the cell-theory along physiological 

 lines ('58). It was Virchow who put on a working basis Schwann's 

 conception that the vital activities of an animal are the sum of all of 

 its parts, and that each part, a cell, or, as Briicke suggested, an 

 "elementary organism," performs all of the characteristic activities 

 of life. Thus while the older physiologists were satisfied with the 

 knowledge that the function of the kidney is to secrete urine 

 containing the waste matters of living activity, the modern problems, 

 as Virchow intimated, centre more especially in the inquiry as to the 

 activity of the kidney cells as such. Again, the modern physiological 

 problem of contractility or of nervous action is concerned with the 

 muscle- and ganglion-cell, and is therefore a cell-problem. For 

 investigations upon cellular physiology there are obvious advantages 

 in studying the unicellular organisms, which, says Verworn, " seem 

 to have been created by nature for the physiologists, for, besides their 

 great capacity for resistance, of all living things they have the invalu- 

 able advantage of standing nearest to the first and the simplest forms 

 of life." 1 



1 Lee ('98), p. 50. 



