212 THE AMERICAN NATURALIST [Vol. LI 



of a circle from the blepharoplast to the point where the 

 undulating membrane terminates. It thus follows closely, 

 in the body plasm, the trend of the membrane, and is re- 

 garded as representing a kind of supporting structure. 

 The chromatin line is heavier in its mid part and tapers 

 at each end. As will be pointed out later, when in the 

 process of spore formation, the trophozoites round off, 

 the chromatin line becomes bent into a hoop, so that its 

 extremities come very near to meeting (Fig. 4). 



Another structure which appears with distinctness in 

 stained preparations is the line of chromatic blocks (Figs. 

 3, 4). These peculiar bodies appear as a single or double 

 row, or as a somewhat irregular line, of deeply staining 

 granules extending from the region of the blepharoplast 

 backward through the plasm to end somewhere in the 

 posterior quarter of the cell. The anterior portion is 

 likely to be thicker and sometimes may partially obscure 

 the nucleus. The curve followed by the line of blocks is 

 about parallel to that of the chromatic line and the two 

 are seldom far distant from one another. 



It is interesting to observe in connection with all of 

 these structures that, in their arrangement, they produce 

 in the flagellate organism a more or less perfect bilateral 

 sjTnmetry. The normal swimming position of the tropho- 

 zoite is with the undulatory membrane above. Directly 

 below this extends the chromatic line and below the chro- 

 matic line is the ''line of blocks." The cytostome is in 

 the midline and somewhat ventral. The blepharoplast is 

 in the midline except in some of the stages of division. 

 The food vacuoles occupy a variable position, but are usu- 

 ally grouped near the middle of the posterior body and 

 caudad of the chromatic line. Sometimes it appears as if 

 the line of blocks and the axostyle passed through the 

 food vacuoles. The axostyle projects from the cell body 

 in the midline although not necessarily at the most poste- 

 rior part of the body. This symmetry is easily seen when 

 the organisms are observed swimming freely in a favor- 

 able medium. Owing to the fact that the dorso-ventral 

 diameter is greater than the transverse diameter, most 



