246 



THE FORMS OF CELLS 



[CH. 



these minute organisms are protean in their changes of form. 

 A certain species will not only change its shape from stage to 

 stage of its Uttle " cycle " of life ; but it will be remarkably different 

 in outward form according to the circumstances under which we 

 find it, or the histological treatment to which we submit it. Hence 

 the pathological student, commencing the study of bacteriology, 

 is early warned to pay little heed to differences oiform, for purposes 

 of recognition or specific identification. Whatever grounds we 

 may have for attributing to these organisms a permanent or stable 

 specific identity (after the fashion of the higher plants and animals), 

 we can seldom safely do so on the ground of definite and always 

 recognisable for7n : we may often be inclined, in short, to ascribe 



Fiw. 73. A flagellate "monad," Distigma 

 proteus, Ehr. (After Saville Kent.) 



Fig, 74. Noctiluca miliaris. 



to them a physiological (sometimes a "pathogenic"), rather than 

 a morphological specificity. 



Among the Infusoria, we have a small number of forms whose 

 symmetry is distinctly spherical, for instance among the small 

 flagellate monads ; but even these are seldom actually spherical 

 except when we see them in a non-flagellate and more or less 

 encysted or "resting" stage. In this condition, it need hardly be 

 remarked that the spherical form is common and general among 

 a great variety of unicellular organisms. When our little monad 

 developes a flagellum, that is in itself an indication of "polarity" 

 or symmetrical non-homogeneity of the cell ; and accordingly, we 



