THE H.^MOFLAGELLATES. 171 



observation so far is to show that the parasites are^ as a 

 rule, never abundant, but, on the contrary, usually rare,^ — at 

 least, in the Trypanosome form.^ 



One reason for this numerical scarcity is, undoubtedly, the fact that 

 multiplicative stages are hardly ever met with. Multiplication of the 

 Trypanosomes, as such (although in all cases, so far as is known, facultative), 

 appears to be generally held in abeyance, except for a short period at the 

 commencement of the infection. Tliig has been well shown by Lave ran and 

 Mesnil (40) in the case of T. lewisi of the rat. After inoculation of an 

 uninfected animal with these parasites multiplication goes on briskly for a 

 time, but then slackens and finally ceases. In ordinary, naturally infected 

 rats, multiplication forms are scarcely ever seen. Sivori and Lecler (106) 

 alone have described them from very young sewer-rats, doubtless only just 

 infected. Similarly in the case of the great majority of Trypanosomes of 

 cold-blooded Vertebrates, dividing forms are extremely rare in naturally 

 infected hosts. The two parasites of the owl appear, however (according to 

 Schaudinn, 1. c), to behave differently in this respect. At stated intervals 

 rapid, successive multiplication goes on for a short time, after which a 

 period of rest and growth ensues. The reason for this is probably to be 

 sought in connection with the fact that neither of these forms undergoes 

 multiplication when in the Hajmosporidian phase (i. e. as a Halteridium 

 or a " Leucocytozoon"), this absence of "schizogony" being a quite ex- 

 ceptional occurrence, up till now, among Hffimosporidia. 



The Trypanosomes in the active motile form are, of course, 

 always free in the blood-plasma (interglobular). But 

 it can no longer be maintained that none are ever, at 

 any stage of life, ecto-^ or endo-globular. Of the two 

 parasites in the owl, one (Trypanomorpha) comes into in- 

 timate relationship with the ei-ythrocytes, and the other 

 (Trypanosoma ziemanni) with the leucocytes and also 



^ It need hardly be again pointed out, that in unusual circumstances, such 

 as an unusually strong infection, mal-nutrition of the host, or captivity, 

 the Trypanosomes may overrun even a tolerant host, and give rise to (in 

 more or less degree) the general symptoms of a pathogenic case. Such 

 instances are noted by Leger (64), Plehn (84), and Hofer (' Allg. Fischerei 

 Zeitung,' xix, p. 48, 1904). 



- See, however, below (Section IX b) relative to the possibility of their 

 occurrence in a Hgemosporidian guise. 



^ The term ectoglobular is here used to denote superficial attachment 

 to the blood corpuscle. 



