N. H. SWELLENGREBEL 
349 
to divide becomes elongated and the chromatin is accumulated at the 
two poles (Diagram II, Fig. 1). At times the achromatic substance 
presents a distinct fibrillary structure; if a karyosomatic granule is 
pre.sent it divides also (Fig. 2). The nucleus becomes more and more 
elongated and shows a constriction in the middle; occasionally some 
chromatic substance may be observed in the neighbourhood of this 
constriction representing perhaps a rudimentary equatorial plate 
(Fig. 3). The final stage of the division is shown in Figs. 4 and 5, 
where a thin filament still unites the two daughter nuclei. 
There is no close relation between nuclear and cellular division; 
a trophozoite may contain several nuclei before the beginning of cellular 
division. 
Diagram I a shows three individuals placed in close contact; this 
becomes more apparent still in the trophozoites figured in Dia¬ 
gram III hh. Whereas Diagram III a shows a state of diffuse 
infiltration, the nuclei of the connective tissue being found between 
the parasites, we see that there is no place left for any nucleus 
between the trophozoites shown in Diagram III hh. These individuals 
form two compact masses which I will call “ aggregations ” in the 
course of this description. This process of aggregation is important 
because it is the beginning of encystment, as will be shown later on. 
Sometimes the trophozoites are surrounded by a thick capsule of 
connective tissue, arranged in concentric layers (Diagrams IV, V and 
PI. XVIII, fig. 5 a). Large nuclei are found situated so near the 
trophozoites that the latter might be regarded as intra-cellular parasites 
(Diagram V a, h). These encapsuled trophozoites show the same 
tendency to aggregation as the individuals of larger accumulations; 
the nuclei of the host’s tissue disappear from the interstices between 
the trophozoites, the latter are closely joined together. 
(6) Formation of the pansporohlasts. When a number of indi¬ 
viduals combine to form an aggregation as do the individuals shown in 
Diagram III hh, they soon become surrounded by a membrane, thicker 
than the pelliculae of the trophozoites. I cannot exactly explain how 
this membrane is formed: perhaps it is constituted by the thickened 
pelliculae of the individuals situated at the periphery of the aggrega¬ 
tion ; if this supposition is right the membrane becomes very soon 
emancipated, as is shown in Diagram VII. 
The number of individuals combining to encyst is very variable. 
In small cysts they are few in number (Diagram VII h), whereas in 
large cysts there are hundreds of them. 
