THE GENUS SPIROCHETA AND ALLIES 231 



the disease to man; after infection, the human victim is first pros- 

 trated in from three to five days ; after the onset, the blood is capable of 

 infecting a mosquito again only for a period of three days. These facts 

 indicate that the organism undergoes some cycle of activity in the 

 mosquito; that it has a period of incubation in man, and that it dis- 

 appears from the blood after three days (see Reports of Yellow Fever 

 Commission, 1900, 1901; also Goldberger, 1900). In spite of all that 

 is known about yellow fever, the organism causing it has never been 

 seen ; it passes readily through the finest filters, and must, therefore, be 

 extremely minute, possibly justifying a position in Borrel's group of 

 the ultramicroscopic or invisible organisms. It may be pointed 

 out, however, as Schaudinn does, that known forms of spirochetes 

 become progressively smaller with successive divisions, and it is con- 

 ceivable that spirochetes consisting of a single unmeasurable node 

 may exist and multiply without forming catenoid colonies in the blood, 

 and, because so minute, remain unseen. Stimpson's ('06) discovery 

 of spirochetes in the kidney of a yellow fever victim is interesting and 

 suggestive in this connection, but they must be found more often 

 before much importance can be attached to them. 

 --G,-Are-S{ttFodietTes"Protozoa or Bacteria? — From the foregoing 

 review of the structures and life histories of the spirochetes there is 

 little that is definite to determine the natural afiinities of these spirilli- 

 form organisms. The plastic nature of the body and polymorphism 

 are protozoan characters. The structure of the so-called flagellum is a 

 point in favor of the bacterial nature, but the highly kinetic membrane 

 is an equally strong point in favor of the protozoa. The nucleus or 

 its equivalent is more like that of the bacteria than like the mor- 

 phological nucleus of the protozoa; but there are protozoa with dis- 

 tributed nuclei (p. 29), so that this character is not distinctive. The 

 physiological characteristics are quite as typical of protozoa as they 

 are of bacteria; division, so often a subject of acrimonious and con- 

 tradictory statements, is not decisive, for many protozoa divide trans- 

 versely (all ciliates and Oxyrrhis and Polykrikos among flagellates), 

 while some bacteria are said to divide longitudinally. Cultivation on 

 artificial media, thus far unsuccessful with spirochetes, is now, thanks 

 to the excellent work of Novy and MacNeal and their followers, no 

 longer a distinctive feature, for trypanosomes, like most bacteria, may 

 be so cultivated. The results of plasmolysis, urged by Novy and 

 Knapp ('06) as an argument in favor of the bacterial nature of spiro- 

 chetes, have but little value, for the time factor necessary to plasmolyse 

 is a purely relative matter dependent upon the nature and resistance 

 of the cell membrane. Differences among the bacteria themselves, 

 in this respect, as Prowazek, Siebert, and many others have pointed 

 out, are quite as marked as the differences between undoubted pro- 

 tozoa and spirochetes. The periodicity of symptoms in the hosts of 



