Opalina. 295 
Can the heterogamous copulation described be abnormal ? 
When I first saw heterogamous copulation in Opalina, it seemed 
possible that it might be abnormal, for several reasons: —- 1) be- 
cause it was observed in material that had been about three hours 
in a Slide culture; 2) because the cysts from which the infection 
was secured had not lain in water before being fed to the tadpoles; 
3) because there were present in the culture individuals of different 
sizes which {had passed unencysted through the alimentary canal 
of the tadpole to its rectum. All doubt however was later removed. 
One of my best series of infections was secured from cysts which 
lay 36 hours in water until all the unencysted Opalinae with them 
were killed. When opening, under the microscope, the recta of the 
tadpoles infected from these cysts, I very often immediately found 
typical heterogamous copulating pairs in different stages of copu- 
lation. Indeed nearly half of the drawings of gametes and zygotes 
of O. intestinalis on the accompanying plates were made from this 
series of infections. Other good infections were made in the same 
manner, giving similar results. Six instances of heterogamous co- 
pulation have since been found in a series of sections of the rectum 
of a tadpole infected sixty hours with O. intestinalis (Figs. 183—185, a, 
Pl. XXIV). 
Encystment following copulation ? 
NERESHEIMER has described encysted individuals which he regards 
as zygotes. He believes that encystment normally follows copulation. 
The zygote cysts, according to his description, are of the same size 
as the infection cysts, but are distinguishable by their usually 
spindle-shaped nuclei, by the fact that the animals more nearly fill 
the cysts, and by the faint concentric striation of the contained body. 
NERESHEIMER saw individuals; containing two spindle-shaped nuclei 
become quiescent, change to oval form and throw off part of their 
cilia, and he interprets these changes as the early phenomena of 
encystment. ENGELMANN had already described for O. ranarum cysts 
with large nuclei in the rectum of the tadpole. ZxntuEr described, 
for the multinucleated Opalinae, multinucleated cysts in the rectum 
of the frog and uninucleated cysts with large nuclei from the rectum 
of the tadpole. Influenced by the observations of these three stu- 
dents, I fully expected to find encysted zygotes in the recta of the 
tadpoles, but J have not seen them for any of the three species 
studied. 
