August 18, 1905.] 



SCIENCE. 



197 



produce an adult tapeworm in the human 

 alimentary canal. The embryonic round 

 worms in the human blood must be drawn 

 into the stomach of the mosquito, wander 

 out into the thoracic muscles and grow to 

 a definite stage of development before they 

 can again enter the human host and become 

 sexually mature adults which produce the 

 blood-inhabiting embryos. In the case of 

 malaria, the germs of Plasmodium malarice 

 must be drawn up into the stomach of the 

 Anopheles mosquito, and within the body 

 of this new host undergo a complicated 

 series of changes before the new generation 

 of spores is ready to be injected with the 

 saliva into the blood of a man in whom 

 these germs produce a new case of malaria. 

 Not only is the intervention of a biting 

 insect essential, and we know none other 

 than the Anopheles mosquito which can 

 'fill the bill'— if you will allow the appar- 

 ently appropriate expression — but it is 

 equally true that the organism must pass 

 through the complicated phases of its life 

 history in the mosciuito before the latter 

 can infect. This is possibly even clearer 

 in the case of yellow fever, even though the 

 specific organism which is the cause of the 

 disease remains as yet unknown. The 

 mosquito which can transmit this disease 

 is also a specific type, Stegomyia fasciata, 

 designated often as the yellow fever mos- 

 quito. It acquires the power to transmit 

 the disease by feeding on the blood of a 

 yellow fever patient, but it can infect a 

 non-immune person only after a period of 

 ten to twelve days. Before that time the 

 bite of this infected mosciuito is harmless, 

 and this condition can be explained only 

 on the basis that the organism of the dis- 

 ease passes through certain stages in its 

 development within the mosquito as a 

 necessary preliminary to reaching the con- 

 dition in which it is able to reenter the 

 human frame and infect such persons as 



are susceptible. Until this period in the 

 life history of the disease germ has been 

 completed, the mosquito remains innocuous. 

 On no other basis than this can the time 

 interval be explained during which the 

 mosquito does not transmit the disease, 

 while after that limit has been passed the 

 insect remains capable of infecting man 

 up to the end of its existence, or at least 

 more than two months. 



The cases given illustrate in a repre- 

 sentative way the phenomenon of alterna- 

 tion of hosts as it occurs often in the life 

 history of parasites belonging to different 

 groups of anin:}als. In some cases the stay 

 in the intermediate host is merely the occa- 

 sion for growth and metamorphosis, as 

 with the blood filarige in the mosquito or 

 the tapeworm embryo in the beef. But 

 in other cases there is a reproductive period 

 in this intermediate host, so that the change 

 of hosts is associated also with alternation 

 of generations or metagenesis. By means 

 of this new generation the number of 

 spores, eggs, embryos or other infecting 

 units is markedly increased and the compli- 

 cated and dangerous life cycle of the para- 

 site finds its compensating factor in multi- 

 plied numbers. Among the arthropod para- 

 sites alternation of generations and change 

 of hosts are rare ; but among the parasitic 

 worms both phenomena occur frequently. 

 Thus all endoparasitic flukes, so far as the 

 life history is known, manifest alternation 

 of hosts and of generations ; direct develop- 

 ment has not yet been shown to occur in 

 any tapeworm, although there is only rarely 

 any new reproductive period in the life 

 cycle. The round worms, or Nematoda, 

 display every grade from the most extreme 

 simplicity and directness of development 

 and transfer to such complicated changes 

 and wanderings as have even yet eluded 

 the scrutiny of the closest investigator, or 

 when announced have aroused the ridicule 



