568 



A Camel Trypanosoma 



points out that Bruce's method is open to criticism. The contention advanced 

 here is that such a criticism, though doubtless true as regards geometrical 

 exactitude, is irrelevant as regards its practical application. The errors 

 involved are so small that they can have no serious effect in the matter of 

 diagnosis, when all the other aspects of the question are considered. 

 Dr. Stephens does not apparently criticise the method of preparing the slide 

 adopted by Bruce, he merely asserts that the actual estimate may err within 

 fractions of a micron. Surely such a source of error cannot be very 

 important in plotting out a curve whose unit is 1 fi, considering, for example, 

 that all values between 5"5 fj. and 6*5 fi will necessarily be registered as 6 fi. 



Again, there are other considerations which tend to make the biometric 

 method of diagnosis at best inconstant : — 



(1) Fixation will vary in different parts of the same slide and in the 

 preparation of slides by different observers. 



(2) Numerous varieties of strain exist among trypanosomes of any species. 



(3) The great similarity between many so-called species as regards their 

 length variation. 



(4) Probably continued maintenance in laboratory animals leads to slight 

 alterations in the morphology of a strain, which to be kept true shoidd 

 from time to time be passed through its insect host. Thus the age of a strain 

 after its last passage through the appropriate intermediate host may be a very 

 important factor in determining its morphology. 



In the face of such objections it would appear that, although the biometric 

 method may have a certain value in diagnosis, such refinements as suggested 

 by Dr. Stephens are by no means essential. What is required is that 

 measurements be made in as constant a manner as possible. Dr. Stephens' 

 method, involving as it does a complicated projection apparatus, is quite 

 impossible for workers in the field. This in itself is an objection which 

 would seriously neutralise undoubted advantages. It is, however, difficult 

 to believe that the ultimate results of the application of both methods to 

 the same slide would differ to any appreciable extent. Granting, however, 

 for the sake of argument Dr. Stephens' contention, it must still be 

 remembered that its occasional employment will introduce yet another 

 variable factor into this method of diagnosing trypanosomes. 



