Opalina. 277 
coming very minute, a few individuals retain nearly their full size and 
do not encyst, but remain in the rectum of the frog, apparently con- 
tinuing the infection of the host. The minute individuals encyst, 
pass into the water with the foeces of the host, and are eaten by 
tadpoles, in whose alimentary canals the little Opalinae work their 
way ont of the cysts and divide, forming micro- and macrogametes 
which copulate. *) 
Decrease in the number of chromosomes. 
In the later mitoses before encystment one finds but four 
(Fig. 120, Pl. XXII) instead of eight (Fig. 119, Pl. XXID) chromo- 
somes. This change in the number of the chromosomes takes place 
in animals from four to eight times as large as the individuals which 
enter the cysts. It occurs before the vegetative chromatin is thrown 
out of the nucleus, the latter process, under normal conditions, 
taking place just before encystment and in the cysts or in the 
rectum of the tadpole. ‘The decrease in the number of the chromo- 
somes might be due either to their union in pairs (synapsis of 
Montcomery) or to an actual “reduction division” at this stage. 
The chromatin ribbon breaks into eight instead of sixteen parts, 
and these do not seem to be unusually large. Possibly counting 
the granules in these chromosomes would show whether they are 
double or not. I have not yet done this, such work being necessa- 
rily slow, and my material needing restaining before any such counts 
can be made. It will probably be necessary to wait until another 
spring in order to have sufficient favorable material for studying 
this interesting point. The reduced number of chromosomes persists 
until copulation occurs (cf. Fig. 148—152, Pl. XXII; 168, 173, 
Pi) XX; 183, 185, 187—191)' Pl? XXTV): 
The last division before encystment. 
One sees very clearly that in the last (?) division by which uni- 
nucleated animals ready for encystment are formed no mitosis of the 
ordinary type occurs (Figs. 121, 122, Pl. XXII). The nuclei seem 
not to be in division at all, but rather are occupied in getting rid 
of a part of their chromatin, a process which will soon be described. 
*) It seems necessary to accept the german use of the word copulation to 
denote fusion of two gametes to form one zygote, and of the word conjugation to 
denote the mutual fertilization of two gametes each by the other, as in higher 
Ciliata. The natural English use of these words would he the exact reverse. 
19* 
