276 JOHNSON. 
reduction in the number of nuclei before encystment and 
spore-formation, but the manner in which this was brought 
about unfortunately could not be discerned, owing to the 
increased opacity of the endoplasm. At a later stage in 
the spore-formation, he says : 
“Jedes Theilstiick verdichtet sich an seinem Umfange 
und umgiebt sich mit einer zarten membranartigen Hiille. 
Innerhalb derselben geht jedes der immer mehr zusammen- 
schrumpfenden Theilstiicke eine Zwe2thetlung ein. Die 
hierdurch entstehenden Halften enthalten je einen Kern, 
sind gewohnlich von der Gestalt eines Kugelsegmentes und 
liegen zuerst platt zusammen, riicken aber bald etwas aus- 
einander, um sich dann wieder zu nahern und mit einander 
zu verschmelzen.’ Here, again, there is uncertainty regarding 
the fusion of the nuclei. 
It will be seen from the foregoing that there is abundant 
opportunity for karyogamy to take place at time of encystment. 
It would be fertilization of the closest kind, for the nuclei 
would have belonged originally to a single cell. But the 
intimate character of the amphimixia would be largely mitigated 
by preceding plastogamy, bringing nuclei from different indi- 
viduals into close propinquity, and thus making possible their 
subsequent fusion. 
In some of my cultures I made the interesting observation 
that the possibility of plastogamy absolutely determined whether 
the colony should survive. As already stated, the only food 
given was Bosmina. After repeated divisions, the Actinos- 
pheria became so small that they could no longer capture 
the Bosminas, and the only possible escape from starvation 
was coalescence. In this way the perpetuity of the colony 
was often assured. It may be claimed that a variety of food 
would be obtainable under natural conditions, and therefore 
preclude the occurrence of such an emergency; but even in 
my aquaria, where other food was obtainable, I seldom saw 
any prey taken by Actinospherium, except Bosmina. 
UNIVERSITY OF CHICAGO, April 26, 1893. 
