Aug. ts, 1923 
Twinning and Monembryony in Platygaster hiemahs 347 
ment of the 2 unfertilized eggs; assuming, of course, that the fertilized 
eggs produced females and the unfertilized eggs produced males. If 
there had been 3 males instead of 2 to account for from the 2 unfertilized 
eggs, 1 of the eggs might have developed twins while the other developed 
a single male by the process of monembryony. 
Admittedly, the brood taken above as an example is only one of many 
ratios of the sexes in which the adult parasites are bred from host puparia. 
But just as the number and the sex of the individuals reared from different 
host puparia are found to vary, so will the factors underlying the develop¬ 
ment of the individuals of a brood vary. These factors are (1) the number 
of eggs in the group deposited in the host, (2) the number of eggs insemi¬ 
nated, (3) the percentage of the eggs becoming aborted, and (4) whether the 
eggs that develop will follow the twinning process or will develop monem- 
bryonically. A study of many combinations of these factors, as found 
in various sectioned host eggs and young larvae, 9 leads to the conclusions 
(1) that mixed broods originate from fertilized and unfertilized eggs de¬ 
posited in the host at the same time by a fertilized parasite, and (2) that 
the monembryonic development of some of the eggs accounts for the 
rearing of one, two, three, or four males or females in a mixed brood in 
which the number of the individuals of the opposite sex predominates. 
BLASTULA TO LARVA STAGES 
To follow the detailed development of the parasites from the blastula 
to the larva stages is in itself an extensive problem, and only a very gen¬ 
eral account of this development can be given here. 
The parasites continue their development from the blastula stage 
during the fall months (PI. 4, D, E), and early winter finds them in the 
advanced embryo stage in which they exhibit larval characteristics (PL 
4, F; PI. 5, A, B). During this interval all of the embryos which have 
arisen by the twinning process become separated structurally from each 
other, although a twin pair may still be located side by side (PI. 5, A). 
Each embryo is found in an embryonic cavity the outer lining of which 
is the trophamnion. During the development of the embryo the troph- 
amnion is relatively thick and contains many small (PL 4, E) or two 
large conspicuous paranuclear masses. The parasites pass the winter as 
well-formed embryos distributed between the fat bodies of the host, 
which has meanwhile become fully grown and encased in a puparium on 
the wheat plant. 
In spring the parasites continue their development. The embryos 
straighten out from their previous U-shaped form and are recognized as 
young larvae. While this development is taking place, the trophamnion 
becomes relatively thinner and its paranuclear masses are absorbed until 
it is represented merely by a very thin membrane surrounding the young 
larva (PI. 4, G). 
In the latter part of spring, the young larvae rupture the trophamniotic 
membrane and begin to feed upon the body content of the host. By the 
time the entire content of the host is devoured, the larvae are full-grown. 
The larvae remain within the body wall of the host during early summer, 
when each larva forms a chamber or cell in which it transforms to a 
pupa. The formation of the pupal chambers, which correspond in size 
to that of the larvae, distends the body wall of the host abnormally, the 
9 The preparations originated from host eggs which had been oviposited in by parasites under laboratory 
controlled conditions. The parasites were reared to the advanced blastula stage in the laboratory. 
