Birge and Juday—Development of Cyclops . 
3 
The number of these cocoons is very great, although no ex¬ 
act estimate can be given, because it is practically impossible 
to ascertain the area from which any given quantity of mud 
has been taken. They seem to lie on the surface of the mud, 
and the largest number is secured if the hose through which the 
mud is pumped barely reaches its surface. If the hose is per¬ 
mitted to enter the mud, few or no cocoons may be found in the 
catch. S’ome idea of the number may be obtained from the 
fact that it was very common to find from four to eight or ten 
in the amount of mud which would be picked up in a medicine 
dropper and spread out for examination in a small watch glass. 
Observations were made from time to time during the summer, 
which showed that the cocoons were present and in the same 
condition as earlier in the season. Since the absence of oxy¬ 
gen excludes the Crustacea from the lower water of Lake Men- 
dota during July, August and September, and since numerous 
examinations of the lower water were made without finding 
young Cyclops in transit from the bottom to the habitable 
water, it seems clear that the cocoons are not hatched during 
this period. It is certain that a considerable number die, since 
the cocoons may be found with the dead Cyclops enclosed. 
Careful watch was kept at the time when hatching might be 
expected—late September and early October. Cocoons were 
found more sparingly in September, at which time also a few 
young Cyclops were taken in the lower water. This was be¬ 
fore the oxygen reached the bottom and indicates that the 
Cyclops were hatching and passing into the habitable water. 
Cocoons were found as late as October 15, and a single cocoon 
was found on October 23. A great amount of material was ex¬ 
amined on that date but no more specimens were discovered. 
In middle and late October the lower water is fully occupied 
by Crustacea, and it is not possible to identify those individuals 
which may have been hatched from cocoons. The facts, how¬ 
ever, show that the resting period is about four months in 
length. 
Since the lower water of Lake Mendota loses its oxygen in 
summer, it was thought that encystation might be induced di¬ 
rectly by the decline in that gas. Observation showed, how- 
