538 



Miss M. Robertson. Polymorphism of 



[July 5, 



importance and the connection with one another of the various instances just 

 cited hardly needs to be further emphasised. 



It has now been shown that the short forms are definitely capable of 

 surviving in the transmitting host, but that in itself does not exclude the 

 intermediate and long forms from a similar development. This apparently 

 does not occur. 



The days of the depressed period continue to be positive for fly until just 

 before the rise, that is until the appearance of the intermediate forms in 

 numbers sufficiently large to cause a serious diminution in the short forms. 

 This period before the rise is one in which the percentage of plus flies produced 

 sinks so low that none are found in the experiments — the number of flies ranges 

 from about 45 to 100 in each experiment. In some cases this is probably an 

 absolute negative, more often it will be only a relative negative, and if several 

 hundred flies were fed probably an infected individual would be obtained. 



The positive and negative periods in regard to the infectivity to fly shade 

 insensibly into one another, thus there is a moment when the drawing out in 

 length of the trypanosomes has begun which is still positive although the 

 negative period supervenes in a short time. When divisions are actually taking- 

 place the blood is infective to fly just in proportion to the number of short 

 forms present. A study of the tables and the analysis of the experiments 

 given later on make this point clear. Such a multiplicative period may have 

 > a negative phase as soon as the proportion of intermediate and long forms 

 preponderates unduly. 



At the height the infectivity has a tendency to diminish, although the 

 requisite form is present in large numbers. This is very marked in a swarm- 

 ing infection showing very numerous parasites, here the swarming period 

 is often negative — it has been a common experience to feed cages even 

 for two consecutive days on a monkey in this state without producing a 

 single infected fly. The reason is pretty clearly that the trypanosomes are in 

 an exhausted state physiologically, and force is added to this when it is noted 

 that the actual period of a drop is negative. 



This last fact is also an argument in favour of the surviving trypanosomes 

 found in the depressed period having developed their resistance during the 

 unfavourable time. That is to say these surviving trypanosomes found 

 immediately after a drop are not, as it were, a separate type of trypanosomes 

 of a resistant character, but are a certain number of the ordinary adult 

 type which have been capable of adapting themselves to the unfavourable 

 conditions at the time of their occurrence. Were this not the case, both 

 the swarming period and the period of the drop would be as infective to fly 

 as the depressed period. 



