Aug. 25,1923 Twinning and Monembryony in Platygaster hiemalis 345 
permit full development of the embryo at the expense of, but not to 
the detriment of, the host. 
Twinning and specialized polyembryony. —In its twinning form of 
development, Platygaster hiemalis exhibits the simplest type of poly- 
embryony, in that only two individuals are developed from a single egg 
or parasite body. Other polyembryonic Hymenoptera, such as Copido - 
soma truncatellum and Copidosoma gelechiae, develop 1,500 to 2,000 and 
150 to 225 individuals, respectively, from a single egg. In these species 
the embryonic region does not divide at the end of the second cleavage 
of the segmentation nucleus, but instead the segmentation nucleus con¬ 
tinues to divide within the original embryonic region, until it is a veritable 
syncytium composed of 200 or more embryonic nuclei. Then at a later 
stage, the trophamnion surrounding the embryonic region penetrates 
among the embryonic nuclei, and after portions of the trophamnion 
encompass one or two embryonic nuclei, the typical germs are formed. 
Disjunction of the many germs then takes place in a manner similar to 
that in the twin germs of P . hiemalis , but on a larger scale. 
Platygaster hiemalis further differs from Copidosoma gelechiae and C. 
truncatellum in that the germ stage and later stages of hiemalis do not 
divide, while the morula stages of both gelechiae and truncatellum do 
divide. It has been shown by Leiby (2) that the morulas of gelechiae 
divide once, each daughter component finally developing into a parasite 
larva. The senior writer has also observed that the morula stage of 
truncatellum divides once and that the daughter morulas also divide, 
each tertiary morula giving rise to a parasite larva. Hence it is evident 
that the division of the morula stage in gelechiae and truncatellum is 
simply a specialization of the twinning process exhibited by hiemalis . 
This specialization by division results in the production, from a single 
egg, of a greater number of parasite individuals which the larger host is 
able to mature. 
Aborted eggs and pseudo-forms. —As previously pointed out, of 
the six to eight eggs deposited in the host at the same time by Platygaster 
hiemalis , approximately one-third fail to develop beyond the segmenta¬ 
tion nucleus stage, apparently because such eggs fail to become invested 
by host tissues. Such eggs degenerate. No stages beyond the egg ever 
degenerate, however, because the twinning and monembryonic processes 
of development are relatively simple. On the other hand, the complex 
and more specialized development of the parasite body of Copidosoma 
gelechiae and that of C. truncatellum results in the production of some 
degenerate germs, embryos, and larvae 7 in these species, along with the 
normal forms that are destined to mature adult parasites. 
It is probable that the development of other polyembryonic species 
will be described in the future, in which it will be shown that four, eight 
or more larvae develop from a single egg. Such species will doubtless 
show that the number of degenerate or pseudo-forms of the embryonic 
stage developed will increase in proportion to the number of mature indi¬ 
viduals originating from a single egg. 
Monembryony and the origin of mixed broods. —The origin of 
single individuals of a sex different from that of all the other individuals 
reared in the same brood has been of some interest to those who have 
reared polyembryonic insects. Especially is this true of the species of 
7 These have been described by Leiby (2) in Copidosoma gelechiae as pseudogerms, pseudomorulas, pseud- 
embryos, and pseudolarvae. Similar forms have been observed by the senior writer in C. truncatellum . 
