Birge and Juday—Development of Cyclops . 
5 
The cocoons began to hatch before the end of September and 
almost all had disappeared before October 8. 
So far as these observations go, therefore, encystation and 
hatching are not coincident with any marked physical change 
in the environment, and must be considered as the result of a 
general process of development. This conclusion is confirmed 
by additional observations made in the spring of 1907. The 
season was unusually cold, the temperature of April in Mad¬ 
ison being 4.5° C., and that of May 4.8° below the average of 
the last twenty-nine years. The lakes warmed slowly, and the 
quantity of oxygen in the lower water was unusually large. 
Cocoons were found on May 29, the bottom water containing 
seven cubic centimeters of oxygen per liter, with a temperature 
of 9.6°. It will be noticed that the cocoons were found earlier 
than in 1906, in spite of cooler weather and a greater supply 
of oxygen. 
It should be noted here that Birge is wrong in the statement 
( ? 97, p. 326) that Cyclops brevispinosus is at all times the dom¬ 
inant species of Cyclops in Lake Mendota. The statement was 
based on the determination of adult females taken from the col¬ 
lections and identified by Dr. Marsh. While (7. brevispinosus- 
doubtless furnished the majority of females identified, the great 
mass of immature Crustacea in the spring must belong to (7. 
bicuspidatus. 
A considerable number of cocoons was hatched in the labora¬ 
tory. Those collected June 4 hatched after a week or more; 
others collected June 14, hatched on June 19. Those collected 
in September hatched, on the whole, more promptly, beginning 
to come out within a day. However, there are always great in¬ 
dividual differences in time of hatching, and there have not been 
enough observations made to determine an average period. In 
1907 hatching began within a day or two. The exit of several 
Cyclops from cocoons was observed under the microscope. The 
cocoon was broken at the rear and loosened by violently extend¬ 
ing and flexing the abdomen, and the animal backed out of the 
part which covers the body. It was positively determined that 
no moulting accompanies the release from the cocoon nor does 
any development of the animal take place in the exit. A nurn- 
