348 
Journal of Agricultural Research 
Vol. XXV, No. S 
host integument becoming so thin that the outlines of the parasite cham¬ 
bers can readily be discerned (PL i, D). 
The parasites remain in their chambers in the pupa or adult stage 
throughout the remainder of summer. They emerge as adult parasites 
from the host carcass and puparium in early fall, to deposit their eggs in 
eggs of the Hessian fly, a brood of which emerges at the same time. 
SUMMARY 
(1) Platygaster hiemalis develops both monembryonically and poly- 
embryonically in the Hessian fly, the adult parasites emerging in late 
summer from the host puparium. An average of 6.31 individuals, often 
of both sexes, is bred from each puparium. 
(2) The female parasite deposits a group of from four to eight eggs at 
one oviposition in the egg and occasionally in the young larva of the 
host during the fall of the year. Some of the eggs of the same group are 
inseminated while others are not inseminated. 
(3) During maturation two polar bodies are formed in the egg. These 
unite to form a single polar nucleus in the anterior region of the egg. 
Maturation is identical in fertilized and unfertilized eggs. 
(4) After maturation the female pronucleus fuses with the male pro- 
nucleus to form a cleavage nucleus, which becomes located in the posterior 
part of the egg. The female pronucleus of an unfertilized egg is similarly 
found in the posterior region. 
(5) Part of the egg containing the cleavage nucleus then becomes 
differentiated from the remainder of the egg. This differentiated part is 
the embryonic region, which together with its contained cleavage or 
embryonic nucleus gives rise to one or two embryos. The remainder of 
the egg, containing the polar nucleus, is homologous to the trophamnion 
and paranucleus of previously described polyembryonic insects. Its 
function is to nourish the embryos until they are young larvae and can 
feed for themselves upon the host. 
(6) The embryonic nucleus divides to form two and then four em¬ 
bryonic nuclei. At the same time the polar nucleus divides to form two 
polar nuclei or paranuclear masses and these divide to form four such 
masses. 
(7) The embryonic region of some of the eggs or parasite bodies then 
divides to form two embryonic regions, and one region together with 
two of the four paranuclear masses becomes separated from the other, 
although both may continue development side by side for some time, 
being held together by host tissues. This division of the parasite body 
results in the formation of twin germs, each of which develops directly 
into a blastula, then into a late embryo stage, and finally into a parasite 
larva. 
(8) The embryonic region of other eggs does not divide. Such eggs 
develop a single parasite by the monembryonic process which is similar 
to that described for other platygastrids. 
(9) Approximately one-third of the eggs deposited do not develop 
beyond the cleavage nucleus stage, probably because they fail to become 
invested by host tissue. 
(10) The twinning development in insects here described for the first 
time is a simple type of polyembryony. On the other hand, the monem¬ 
bryonic development of this parasite is very highly specialized. Since 
