THE HA EM O FLA GELLA TES 229 



are rounded forms with both nuclei but no locomotor apparatus 

 comparable to ookinetes, in short (cf. T. barbatuhie above). These 



individuals, says Miss Robertson, which divide in this condition 



fairly actively, gradually disappear from the crop and are found 

 only in the intestine. Here they develop a locomotor apparatus, 

 but persist for some time in a Crithidia-likQ form ; they are of 

 varying size and may be very small. Later on, these individuals 

 take on a more or less typical Trypanosome-like, or, as we have 

 previously termed it, trypaniform character, with the kinetonucleus 

 in the aflagellar half of the body, though its actual position varies 

 greatly. These trypaniform individuals are of two main types, 

 which appear, however, to be connected by intermediate grades. 

 One kind is relatively very broad, with a relatively small kineto- 

 nucleus, but usually with a fairly long flagellum. The other type 

 is a long slender Trypanosome, with a large kinetonucleus, but the 

 free flagellum is not, as a rule, very long. The constitution of the 

 trophonucleus presents an unusual condition ; it is very much 

 drawn out, and the chromatin is arranged in a number of transverse 

 rods or bars (perhaps comparable to chromosomes) arranged more 

 or less parallel, like a ladder (cf. author's note on T. brucii above, 

 p. 216). About the middle of digestion, these Trypanosomes occur 

 chiefly in the intestine, but also in the crop, often in large numbers. 

 At a later period, a still more slender, practically thread-like form is 

 developed, which is met with chiefly in the proboscis, though also, 

 apparently, in the intestine. This type, which differs rather from 

 the last, appears to die off if it remains in the leech, and taking 

 this in conjunction with the occurrence of these individuals in the 

 proboscis, the inference is that this is the form in which the 

 parasites are inoculated into the fish. At the close of digestion, 

 a number of very small forms are always to be seen, either in a 

 rounded (probably resting) condition or in a very early Crithidial 

 phase. These seem to be persistent forms, through which the 

 leech retains the infection. 



Miss Robertson discusses the likelihood of the two contrasting 

 trypaniform types above described representing male and female 

 individuals, but for several reasons hesitates to accept this view. 

 However this may be, it is more probable that conjugation itself 

 takes place soon after the transfer of the parasites from one host to 

 the other, i.e. after the arrival in the Invertebrate ; and that the 

 ookinete form is the immediate result of the process. This is 

 suggested by Leger's work on T. barbatulae, as well as by Keysse- 

 litz's account of the life-cycle of Trypanoplasma borreli (27). It 

 is also regarded by Prowazek (68) as being the case in T. lewisi, in 

 the louse. 



According to Kcysselitz, male and female gametes can be 

 readily recognised in the blood of the fish (carp), the conjugation 



