PARASITES RESEMBLING NORMAL CELLS. 179 



ous forms among the Protozoa which are hardly distinguishable from 

 these naked cells, except by their free life. Of such animals the best 

 known are the Amoebae (Proteus diffluens of the old zoologists), which 

 are often found in crowds in the sediment of fresh and salt water (we 

 shall speak later of an Amoeba parasite in man), and which in struc- 

 ture and function correspond so closely with the naked cells, that we 

 are accustomed to call the latter " amoaboid " cells, or to describe their 

 movement and method of nutrition by this term. 



Nor are these Amoebae distinguishable from cells in the nature of 

 their reproduction. In both this takes place by division, from the 

 nucleus outwards, through the surrounding protoplasm, and results in 

 the formation of two distinct masses out of the originally simple body- 

 substance. A strictly sexual reproduction has not yet been observed 

 with certainty in any of the Protozoa. 



It is further to be noted that the Amoebae are by no means the only 

 Protozoa which are, in structure and function, interchangeable with 

 the cellular elements of the higher animals. The other groups also 

 contain similar forms in greater or less number. 



This resemblance is not without importance in the history of our 

 science. Even in our day certain parasitic Protozoa (which, as we shall 

 afterwards see, are very numerous) have, on the ground of this resem- 

 blance, been often considered as component parts of their host, and in- 

 versely some cells, in reality belonging to the host, have been classed 

 as parasites. As illustrative of this latter case I need only refer to the 

 so-called " Spermatozoa," which, as is well known, were quite gene- 

 rally ranked as parasitic animals till the time of Kolliker, and were on 

 the strength of this idea frequently credited (even by Ehrenberg, 

 Valentin, and others) with more or less perfect internal organs. So 

 deeply rooted was the notion of the animal nature of these bodies, that 

 it still prevailed even when the existence of living spermatozoa had 

 been long recognised as indispensable to semen capable of fecundation. 

 The only concession in consequence of this fact was to regard the sper- 

 matozoa as " necessary parasites." At that time, too, observers further 

 deceived themselves with the thought that the blood corpuscles might 

 be other similar " necessary parasites " (Mayer). 



It is a still more difficult task, as already hinted, to determine the 

 boundary between parasitic Protozoa and some similar parasites of 

 vegetable origin. With regard to the latter, I have principally the 

 Schizomycetes in view, organisms which, on account of their motile 

 powers, were very generally referred to the animal kingdom as Bacteria 

 and Vibriones, although their nearest relations, the Phycochromacese 

 (Oscillator ia, &c.), containing chlorophyll - granules, are decidedly 

 plants. In former times it was the fashion to call everything an 



