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readily detached in the manipulation of the plankton, so much so 
that in 1898 less than 6 per cent. remained attached. More or less 
uncertainty attends the determination of the parentage of detached 
winter and male eggs, so that decisive proof of sexual reproduction 
is best obtained from the attached eggs. In Table I. will be found 
the records of free and attached male and winter eggs recorded in 
1898. Evidence will be found in this of sexual reproduction at- 
tending the pulses of March, April, May, September, and December. 
The presence of winter eggs at intervals throughout the greater part 
of the year may be due either to their continual production or, as 
seems more probable, to their continuance in the plankton for some 
time after their formation. The presence of attached winter eggs, 
or of larger numbers of free winter eggs, seems to mark the culmina- 
tion and decline of the pulse. Male eggs, on the other hand, are 
more generally present during both the rise and decline of the pulses. 
Somewhat similar evidence of sexual cycles attends many of the 
larger pulses in years prior to 1898. 
This species affords a striking example of a perennial eulimnetic 
planktont. It is found in midwinter under the ice in water at the 
freezing point, and even under these conditions it multiplies, pro- 
ducing pulses whose amplitude surpasses that of many rotifers of the 
plankton, and runs a reproductive cycle similar to, though of less 
amplitude than, those at other seasons of the year. It shares with 
other organisms the vernal outburst, and repeats the process in 
summer months under maximum conditions of heat and in waters 
whose chemical condition is very different from that in which the 
hiemal and vernal pulses appeared. Successive generations of this 
species are thus adapted to widely different conditions. Through 
all the changes incident to ice, stagnation, flood, sewage pollution, 
changing temperature, the wax and wane and change of food, the 
constant and unceasing warfare of enemies which prey upon it and 
of parasites which plague it, and, above all and continuously, the 
removal of countless individuals from the place of their origin by 
the ceaseless current of the stream, this species lives on, holds its 
own in the plankton, and repeats year after year the same sequence 
of rhythmic pulses of occurrence in the river water. The secret of 
the process doubtless lies in its capacity to produce repeatedly these 
crops of winter eggs which serve to seed the environment and start 
