344 
Journal of Agricultural Research 
Vol. XXV, No. 8 
within it are not all in the same stage of development. For example, 
one host larva containing three parasite bodies will show (i) that two 
of the parasite bodies are in the twin-germ stage, each embryonic region 
of which contains four embryonic nuclei (PI. 3, H), and (2) that the 
third parasite body contains eight embryonic nuclei in an undivided 
embryonic region (PI. 3, I). An older host larva containing seven germs 
or blastulas will show (1) four blastulas at the 16 or 32 cell stage, which 
by their paired association in the host and the fact that each pair is 
held together by host tissue are known to have developed by the twinning 
process, and (2) three isolated parasite bodies showing 8 or 16 nuclei 
in their embryonic regions, which by their shape (PI. 4, C) and position 
in the host obviously developed from individual parasite eggs. vSince 
twinning of a parasite body takes place only at the 4-cell stage of the 
embryonic region, and since an odd number of germs and blastulas is 
frequently found in the host, the conclusion can safely be drawn that 
some of the eggs of Platygaster hiemalis develop monembryonically. 
The monembryonic process of development in Platygaster hiemalis is 
not unlike that described by Marchal (4) for the platygastrids Synopeas 
rhanisy Trichasis remulusy and Platygaster ornatus. In these species 
Marchal demonstrates how the embryo develops in a differentiated 
region of the egg, just as P. hiemalis does when it develops monem¬ 
bryonically. Moreover, the eggs of the above-mentioned species de¬ 
scribed by Marchal develop in a cyst of host tissue, nourishment from 
which is obtained by the trophamnion and the paranuclear masses and 
supplied to the growing embryo exactly as in P.. hiemalis . 
SUMMARY AND DISCUSSION OF CLEAVAGE TO BLASTULA STAGES 
It has been shown above that after maturation and fertilization 
in the case of an inseminated egg an embryonic region becomes differ¬ 
entiated in the posterior part of the egg, and that after some growth of 
the parasite body, the embryonic region divides, two of the four em¬ 
bryonic nuclei passing to each of the two newly produced embryonic 
regions. Each of these two embryonic regions, together with its com¬ 
ponent paranuciear masses and trophamnion, develops into the blastula 
and finally into the larva stage, thus demonstrating a form of poly- 
embryony in this species known as twinning. Moreover, certain of the 
eggs fail to develop beyond the segmentation nucleus stage and become 
aborted, probably because they do not become invested by host tissues 
from which they could absorb nourishing material necessary for their 
continued development. It has further been shown that some of the 
parasite bodies do not twin, but on the other hand develop monem¬ 
bryonically in a manner not unlike the monembryonic development 
described by Marchal for other platygastrid species. 
Specialized monembryony. —It is obvious that Platygaster hiemalis 
exhibits both the highest type of monembryonic development and the 
simplest type of polyembryonic development yet known. In its monem¬ 
bryonic development, it is specialized to the extent that it must draw 
upon its host for nutriment during the course of development of the 
embryo, for the reason that its egg is too small (its yolk is therefore 
insufficient) to develop a larva to the point where it can feed for itself. 
Consequently the cortex of the differentiated embryonic region is utilized 
as a nutritive membrane (trophamnion), which, together with its para¬ 
nuclear masses (of polar body origin), provides nutriment sufficient to 
