Hyalella Knickerbokeri 247 
On April 15, 1921, I observed a male swimming about with a 
female which he held in the usual way, but every once in a while, when 
swimming he braced his pereopods against the dorsal side of the female 
and then by forcing them backward quickly, produced a short quick 
jerk of himself but did not seem to effect the female. This he did three 
or four times, which actions seemed to be a signal for copulation or at 
least a procedure gone through with before copulation which immediately 
followed and lasted a period of twenty-five or thirty seconds. 
Without changing or releasing his hold of the female with his first 
gnathopods by which he held her on the dorsal side, he extended the 
posterior part of his body around the female to her ventral side until 
his uropods touched the marsupium of the female at its mid-ventral 
part. He pressed the marsupium with a succession of quick movements 
with the tips of his uropods, lasting as stated about some twenty-five or 
thirty seconds. His last thoracic somite just turned past the coxal 
plates of the female so that the ejected sperm could be quickly swept 
into the marsupium by the fast moving pereopods of the female. Mean- 
while the male did not perceptibly move his pereopods. He next straight- 
ened himself into the dorsal position and swam off with the female, but 
modified the swimming by extending his first gnathopods with which he 
still clung to the female, pulled the female back as if he were shaking 
her. He did this several times and the whole process as described above 
was repeated as many as eleven times. The next morning, April 16, 
the eggs were deposited in the brood pouch and he no longer swam 
about with the female. One time IJ observed a pair copulating while the 
eggs were passing down the oviduct but usually, as far as I can observe, 
copulation occurs before the eggs even start down the oviduct. 
On April 12, 1921, I observed a male carrying a female which had 
jaid her first egg in the brood pouch. When the second one had passed 
down she struggled free and from that time 11:12 to 11:52 a. m., a 
total of forty minutes exactly, she had laid all the eggs. My observa- 
tions for Hyalella knickerbokeri is the same as Sexton and Mathews 
(183) for Gammarus chevreauxi as to the laying of the eggs, for the 
last one laid was the most posterior and also as they stated an oviduct 
was clearly seen. I could distinguish it only when the eggs were passing 
down it. 
The ova are of a dark green color and as they left the ovary by 
means of this small tube, smaller in diameter then the ova, they were 
pressed out of their usual spherical shape. The eggs passed down both 
oviducts at the same time. The oviducts were seen to open at the base 
of the fifth coxal plate. 
The incubation period for Hyalella knickerbokeri is, as stated above, 
about twenty-one days. The eggs remain green for a week or ten days, 
then turn black. When examined under the binoculars one can see the 
elongated embryos. About the eighteenth to the twentieth day they 
become a reddish brown or pinkish and on the twentieth day the red 
eyes of the embryo can be distinguished very well. Miss Langenbeck 
(98) says that the colors of the embryos of Microdentopus gryllotalpa 
Costa changed color in a similar manner. 
The young after hatching may be extruded that same day or any 
