380 
PACIFIC SCIENCE, VoL XVIII, October 1964 
vided to the embryos within the brood pouch 
by the mother or lack of ventilation of the 
embryos may have contributed to their weaken- 
ing. Secretion of nutrients by the mother has 
long been suspected for isopods, but no definite 
evidence proves its occurrence, at least prior to 
hatching. Wesenberg-Lund (1939) has ques- 
tioned the production of such nutrient sub- 
stances in fresh-water isopods. Based on length 
measurements of marsupial young, and on ex- 
marsupial cultures of embryos, Lemercier (1957) 
denied the existence of such secretions by the 
aquatic Jaera marina . Saudray and Lamercier 
(I960), on the other hand, showed that al- 
though there was a decrease of vitelline reserves 
( and of dry weight ) before hatching in eggs of 
the terrestrial Ligia oceanica, there was a con- 
siderable increase of weight, in great part from 
minerals, after hatching, and that this increase 
probably was of maternal origin. 
The newly hatched young were by no means 
able to care for themselves. Locomotion was 
impossible; although in the experimental con- 
tainers they appeared to attempt to right them- 
selves after hatching from their (invariable) 
position on their backs, they never succeeded in 
doing so. Specimens that were pushed by the 
investigator into a position with the ventral 
side down always rolled over on their backs 
immediately when the supports were removed. 
Likewise, embryos within the egg membranes 
were almost invariably lying dorsal side down, 
though a few were not. 
After hatching the young remained in the 
brood pouch for a considerable period of time 
(the length of their stay could not be ascer- 
tained). During their sojourn in the brood 
pouch they developed a great deal more pig- 
mentation than the small amount present at the 
time of hatching, and they increased considerably 
in size. Some attained a length of a little more 
than 2 mm, or one-third the length of the 
parent. Liberation of the young from the brood 
pouch was not observed. 
Discussion 
There was considerable increase in size of the 
embryo before hatching, and even more sub- 
sequently. It was not possible to distinguish 
between true growth ( i.e., increase of biomass ) 
and false growth by intake of water. True 
growth could occur only if the mother provided 
some nutrition to the developing young. 
The extremely rapid growth occurring at the 
time of hatching could not have been a true 
growth, for new protoplasm cannot be formed 
so rapidly. It is believed that the size increase 
came about by the absorption of water from 
the environment, and that it probably was as- 
sociated with the liberation of osmovalent sub- 
stances in the protoplasm or the hemocoel at 
this time. 
Experimental tests of the permeability of the 
egg membrane could not be undertaken, but 
the steady growth of the embryo and of the 
postembryonic animal, and the rapid growth at 
the time of hatching, indicate that the mem- 
brane was permeable both to water and to any 
nutrient substances the mother may have se- 
creted into the brood pouch. Originally loosely 
surrounding the egg, the egg membrane later 
became taut and invested the embryo closely. 
This does not suggest that the membrane itself 
was osmotically active, as occurs in hatching 
copepods and in some insects (Davis, 1959, 
1961 ). 
Hence the hatching process appears to pro- 
ceed somewhat as suggested by Przylecki (1921) 
for Daphnia, where the embryo swells ( in 
Daphnia only by the uptake of water) until 
finally the egg membrane is sloughed off. 
Hatching has previously been described briefly 
for isopods by Ellis (1961), who said that in 
Asellus intermedins there is an outer egg mem- 
brane, an inner egg membrane, and a larval 
membrane. The outer membrane splits and is 
shed at an early stage of development. The other 
two membranes are shed later. Nothing is said 
of an increase of embryo size, however. Earlier 
investigators (Verhoeff, 1920; Forsman, 1944; 
Naylor, 1955) mentioned an increase of size. 
Both Forsman and Naylor studied aquatic iso- 
pods (respectively, Jaera albifrons and Idotea 
emarginata) . Forsman said that the first stage 
of life in the brood pouch terminated when the 
egg shell suddenly burst and slipped off. The 
enclosed embryo immediately increased in size 
but was still enclosed in an "embryonic mem- 
brane.” Only after some time the latter suddenly 
ruptured. It is not clear from Forsman’s descrip- 
tion that the size increase caused either of the 
eclosions, but he specifically stated that final 
