Mabch 9, 1906.] 



SCIENCE. 



369 



period of incubation, as well as in the 

 change of hosts, is a striking argument in 

 favor of the protozoon nature of the yellow- 

 fever germ. 



The different forms which protozoa as- 

 sume at various periods of the life-cycle 

 have been frequently mistaken for different 

 species or different genera, and well-known 

 eases such as Plasmodium and Polymitus, 

 or Coccidium and Eimeria, where the dif- 

 ferent phases were regarded for a long 

 time as distinct types of organisms, justify 

 the view that the entire life-cycle should 

 be taken into consideration when describing 

 species. The changes in form at the dif- 

 ferent periods are due, as I believe, to the 

 differences in vitality. 



[Here a number of lantern slides were 

 shown illustrating the variations in struc- 

 ture, etc., in various types of protozoa.] 



In Tetramitus the free-living asexual 

 forms have a definite membrane and a 

 definite body form and continue to multiply 

 by simple division for a period of two or 

 three days, when the definite membrane be- 

 comes plastic and the body assumes more 

 or less amceboid forms resembling the genus 

 Mastigamoeba. In this condition of the 

 protoplasm two individuals upon coming 

 together fuse into a common mass, their 

 nuclei unite and conjugation is effected. 

 After conjugation a firm cyst is secreted 

 within which the protoplasm divides into 

 hundreds of parts, which escape from the 

 cyst, finally, as young flagellates. 



In Polystomella the polymorphism is 

 shown by two types of shell and by differ- 

 ent forms of the young organisms. In the 

 microsphasrie type there is a characteristic 

 fragmentation of the nuclei into many 

 chromatin particles of small size. This is 

 followed by formation of amoeboid spores, 

 which develop into shelled forms of a dif- 

 ferent type from the first (macrosphseric), 

 and these in turn give rise to flagellated 

 spores which conjugate. 



The decrease in vitality can be measured 

 in a rough way and the relative vitality at 

 different periods can be easily compared. 

 This has been done in the case of some of 

 the free living infusoria, for example in 

 ParamcBcium, Oxytricha, etc. For such 

 comparisons a single individual is isolated 

 and within twenty-four hours it divides. 

 The daughter cells are similarly isolated, 

 and this process is repeated until the pro- 

 toplasm under observation dies from ex- 

 haustion of vitality, in one case {Paramoe- 

 cium) after twenty-three months of ob- 

 servation, during which time 742 divisions 

 were recorded. When averaged for ten- 

 day periods these divisions gave a satisfac- 

 tory measure of the vitality at different 

 times during the cycle. It was found in 

 these experiments on Paramecium that 

 after about 200 generations the vitality is 

 apparently exhausted, but that it can be 

 restored by artificial means and stimulated 

 to a new cycle of about 200 generations. 

 Ultimately, however, artificial stimuli failed 

 to renew the vitality and the race died out 

 in the 742d generation. Such artificial 

 stimulation suggests the possibility that in 

 certain human diseases, such as malaria, 

 the organisms may become exhausted so 

 far as the division energy is concerned, but 

 may remain quiescent in the system, hiber- 

 nating, as it were, in some organ until, 

 owing to some change in the chemical com- 

 position of the blood, an artificial stimulus 

 renews the division energy and a recur- 

 rence follows. 



Turning now to the data that have ac- 

 cumulated in regard to the organism of 

 yellow fever, we must note that the rapid 

 development in the blood indicates a high 

 potential of vitality; that the disappear- 

 ance from the blood indicates that the 

 organisms have been killed off through 

 excess of their own toxins, or by accumula- 

 tion and action of the anti-bodies. The 

 long period of incubation in the mosquito 



