368 



SCIENCE. 



[N. S. Vol. XXIII. No. 584. 



vitality with continued division or asexual 

 reproduction, until it ultimately dies of old 

 age no less surely than does the protoplasm 

 that makes up the tissues and organs of 

 higher animals. It is known that a pro- 

 tozoon immediately after conjugation with 

 another of its own kind, a process which 

 agrees in essentials with the fertilization 

 of an egg by a spermatozoon, has a high 

 potential of vitality which enables it to live 

 and multiply asexually for a long period, 

 with, however, a gradually decreasing vi- 

 tality which, unless renewed by conjuga- 

 tion, ultimately gives out, the protoplasm 

 dying of old age. Conjugation, then, ap- 

 pears to be a process of rejuvenescence and 

 has been so interpreted since the classical 

 experiments of Maupas in 1888 and 1889. 



In every protozoon life-cycle, in free 

 living and parasitic forms, we can make 

 out, with more or less precision, three 

 phases which correspond with analogous 

 phases in the life of a metazoon. The 

 vigorous, actively dividing forms are found 

 in the period immediately after conjuga- 

 tion and this period corresponds M'ith the 

 period of youth. The conjugating period, 

 or the time when the protoplasm is capable 

 of renewing its vitality by conjugation, 

 corresponds with the period of adolescence, 

 or maturity, and, in the forms which have 

 not conjugated, the period of increasing 

 degeneration and old age compares with 

 the old age of metazoa. 



The study of many types of protozoa has 

 shown that with decreasing vitality there 

 are frequently marked changes in the form 

 of the body and in the physical composi- 

 tion of the protoplasm. These changes are 

 most marked at the period of sexual ma- 

 turity, when they frequently give rise to 

 structural modifications as widely different 

 as the egg and the spermatozoon. It is 

 while in this condition of the protoplasm 

 that sexual union take place and, through 

 this, renewal of vitality and return to the 



type form. This condition of the proto- 

 plasm, which indicates a modification of the 

 physical organization of the protozoa at 

 periods of adolescence, comes sooner or 

 later in the history of any life-cycle, but 

 it can also be induced artificially in many 

 cases. Thus it has been possible to change 

 ordinary asexual flagellated protozoa into 

 sexual forms by increasing the density of 

 the surrounding medium through addition 

 of sugar, etc. ; or by lowering the tempera- 

 ture, perhaps again, a matter of density. 

 In a number of parasitic forms, some of 

 which affect the welfare of human beings, 

 a similar change in environment is a normal 

 part of the life-cycle, and is brought about 

 by insects, sometimes mosquitoes, sometimes 

 ticks and sometimes flies. The asexual 

 protozoon organisms are transferred from 

 the warm blood of birds or mammals, or 

 man, to the cold environment of the insects' 

 digestive tract. This is accomplished in 

 the case of malaria by mosquitoes belong- 

 ing to the genus Aiwpheles; in the case of 

 bird malaria by mosquitoes belonging to 

 the genus Culex; in the case of sleeping 

 sickness by the tsetse-fly; in the ease of 

 Texas fever among cattle by ticks belong- 

 ing to the genus Boophilus. Where the 

 full life history has been made out, it has 

 been found that conjugation takes place 

 within the body of the insect and here, 

 therefore, vitality of the parasite is re- 

 stored. 



What is known to take place in some of 

 these well-authenticated cases is presum- 

 ably true in the ease of yellow fever. The 

 blood with the organisms in it is taken 

 into the digestive tract of the mosquito 

 (Stegomyia fasciata), and here, or in some 

 other organ, the germ probably passes 

 through some developmental cycle, for a 

 period of twelve days is necessary before 

 such infected mosquitoes can transmit the 

 disease to another human being. The sim- 

 ilarity to the malaria organisms in the 



