﻿Trypanosome found in Pontobdella. 



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frequently shows a very large karyosome, which contains a 

 quantity of a plastin-like substance. It is from these forms 

 (Fig. 15, a) that the large broad trypanosomes are later 

 developed. I am ignorant as to their earlier history, but am 

 inclined to imagine that they arise simply by growth from 

 the ordinary smaller forms. 



Development of the Flagellum. 



From the rounded forms, which, it will be remembered, 

 have a well-defined trophonucleus and a large, persistent 

 kinetonucleus, there arises a flagellate, which is in the 

 condition of a Crithidia or Herpetomonas as regards its 

 flagellar relations. I propose to call this the crithidial 

 stage, to distinguish it from the trypanosome condition. 



There is one remarkable feature in the stage at which the 

 motor organ arises, namely, that this is usually, so far as my 

 experience goes, essentially a division stage. Two flagella 

 arise side by side from two little structures, and division is 

 apparently the usual, if not the invariable, sequel (Figs 28- 

 41). This part of the cycle will be made most clear by a 

 description of the various stages. One of the first stages 

 (I propose to call this stage 3, Figs. 30 and 31) consists of 

 a slightly oval individual, with an elongated but not yet 

 obviously divided kinetonucleus. In front of the kineto- 

 nucleus, and parallel to its long axis, a fine but quite distinct 

 red-staining bar may be distinguished. This bar is bound 

 to the kinetonucleus by four fine red staining threads 

 (Figs. 30 and 34). It may be noted that the red bar and 

 the connecting threads are very constant features. Between 

 the bar and the (anterior) margin of the protoplasm are two 

 slightly irregular spindle or rod-shaped bodies lying close 

 beside each other. I propose, for the sake of convenience, 

 to call these the preflagellar bodies — there is as yet no sign 

 of the flagella themselves. One has no very definite explana- 

 tion to offer as to why these structures should so generally 

 arise in this double condition. There is, however, a sugges- 

 tion, which I advance quite tentatively, that this double 

 origin of the flagellar apparatus may be due to a phylogenetic 



