WILLIAM H. TALIAFERRO 689 



immune-rabbit serum ranging from o.i to 2.5 cc. per 20-gm. mouse. Within two hours, 

 mouse II, which received 0.7 cc. of serum, showed a crisis, i.e., the trypanosomes dis- 

 appeared from its blood. Mice i, 3, 4, 7, 8, and 10 behaved in a like manner, whereas 

 mice 2, 5, 6, and 9 did not have a crisis, did not show a diminution in the number of 

 their trypanosomes, and died as soon as the control. (For many similar experiments 

 see the author and Johnson [1926].) 



At first sight one would be tempted to explain such zone phenomena simply on 

 the basis of host variability. Later work by Johnson (1927) indicates, however, that 

 its occurrence is dependent on the number of trypanosomes in the mice at the time 

 the immune serum is given, as Table II shows. The absolute number of trypanosomes 

 at which the zone phenomenon occurs varies with different single cell strains of 

 trypanosomes. These differences are probably induced and of long duration, but not 

 true genetical variations— similar, in other words, to the acquisition of antibody 



TABLE II 



Relation between Numbers of Trypanosomes and Zonal Phenomena for a 

 Single Cell Strain of T. cquinum (from Johnson) 



Number of Trypanosomes per Standard Microscopic Field 



AT THE Time of Treatment with Immune Serum Occurrence of Zone Phenomena 



1-5 No zone phenomena; all dosss 



greater than minimal effective 

 dose are effective 



9-33 Zone phenomena; doses greater 



than minimal effective dose 

 show alternate zones of effective- 

 ness and non-effectiveness simi- 

 lar to table I 



45-50 No lysis in any dose up to 4.0 cc. 



per 20-gm. mouse 



resistance. Thus, Johnson was able to take strain J4, which gave the zone phenome- 

 non with one to five parasites per field, and by subjecting it to immune serum and 

 securing a relapse strain, obtained the zone phenomenon when there were ten to 

 thirty parasites per field. 



The results of Johnson on the relationship between the number of trypanosomes 

 and the zone phenomenon are in accord with the recent results of Coventry on the 

 action of antipneumococcus serum on pneumococci in which the serum and virus 

 were introduced simultaneously. The reader is referred to Coventry (1927) for a dis- 

 cussion of similar results in the bacteriological literature and for a consideration of 

 some of the hypotheses advanced to explain it. I am of the opinion that no satis- 

 factory explanation has yet been given for these peculiar results in in vivo work. 



INFECTIONS WITH THE NON-PATHOGENIC TRYPANOSOMES 



Trypanosoma lewisi is a representative of a large group of trypanosomes, occurring 

 in various species of rodents, which are very similar in structure and are non-patho- 

 genic to their hosts. Careful enumerative studies of the form by Steffan (1921), fully 

 confirmed by the author and L. G. Taliaferro (1922), the author (1924), and Coventry 



