404 RESEARCH IN PROTOZOOLOGY 



likely to him, there was a double infection with two broods of para- 

 sites, each with a forty-eight-hour cycle. The strain has been kept 

 by direct subinoculation in canaries for over eleven years and this 

 may have altered the synchronicity even if the basic time of the 

 asexual cycle remained unchanged. 



With the cycle and its periodicity established in avian malaria, the 

 next question appeared to be how constant is the cycle. Recent data 

 indicate that it is remarkably constant and correspondingly dif- 

 ficult to change. By refrigerating parasites (P. c at h enter iiim) for 

 six, twelve, and eighteen hours and by injecting very large num- 

 bers into birds intravenously so that studies could be made from 

 the moment of injection, the present author (1928) found that the 

 cycle was delayed by the refrigeration, but that it completed itself 

 in approximately twenty to twenty-two hours, instead of the cus- 

 tomary twenty- four hours, imtil sporulation again occurred at the 

 regular time from which it did not thereafter vary. 



The foregoing work has dealt with the periodicity in asexual 

 forms. Recently, Cowdry and Cowell (1928) have studied blood 

 smears made at four-hour intervals over a three-day period from 

 a monkey infected with Plasmodium kochi. They found that the 

 size of the male and female gametocytes varies more or less syn- 

 chronously during approximately a twenty-four-hour cycle which 

 reaches its peak at about 4:00 p.m., and that the females are gen- 

 erally slightly larger and more numerous. 



The study of the cycle has also yielded interesting results in 

 relation to the acquired resistance of the body to malarial infec- 

 tions. Although it has often been tacitly assumed that an acquired 

 resistance resulting in a decrease in the number of parasites is 

 associated with a decrease in the rate at which the parasites are 

 reproducing, W. H. Taliaferro and the present author (1922) 

 pointed out that a decrease in numbers signifies a destruction of 

 the parasites which may or may not be associated with a decrease 

 in the rate of reproduction per se, and that the two factors must, 

 therefore, be differentiated in any analysis of an acquired resist- 

 ance. Such a study has been undertaken in avian malaria by mak- 

 ing frequent number counts and by comparing the length of the 

 asexual cycle in the various parts of an infection when the para- 

 sites are present. Parenthetically, it may be said that the length of 

 the asexual cycle which is a direct measure of the time it takes for a 

 small merozoite to produce approximately sixteen (in avian ma- 



