Aug. 25, 1923 
Twinning and Monembryony in Platygaster hiemalis 341 
from the remainder of the egg (PI. 2, X). This differentiated region con¬ 
tains the cleavage nucleus, and is called the embryonic region, since it 
later gives rise to the embryos. The remainder of the egg containing the 
polar nucleus constitutes the polar region. 
Between the first and second days the group of from 5 to 8 eggs deposited 
by the parasite in the host becomes somewhat dispersed throughout the 
developing host if they were deposited in a host egg. If the eggs were 
deposited in an embryo well advanced in development at the time of 
oviposition or in a recently hatched host larva, they become scattered in 
the body cavity of the host at an earlier hour. In any event the eggs 
begin to increase in size soon after the first day, when they measure 
18.2 n in length and 7.8 n in width. Since the eggs increase in size, the 
egg stage may be regarded as past and henceforth the eggs may be 
denoted as parasite bodies. 
During the dispersal of the parasite bodies in the cavity of the host 
they become lodged against the host tissues, such as the salivary glands 
or fat tissues, whereupon portions of these tissues soon encompass the 
parasite bodies. Occasionally two parasite bodies may be surrounded 
by the same bit of host tissue, but both parasite bodies will usually con¬ 
tinue to develop independently of each other. After each parasite body 
is surrounded (at least partially) by host tissue, an elaboration of the 
paranuclear masses in the polar region can immediately be observed. 
This elaboration is obviously due to the absorption of host tissues by the 
parasite body through its polar region. The polar region, including that 
part which surrounds the embryonic region, is therefore properly known 
as the trophamnion. 
CLEAVAGE TO BLASTULA STAGES 
The development of the embryos, from cleavage of the segmentation 
nucleus to formation of the blastulas, takes place for the most part in 
the young host larva, or during the interval from the second to the 
tenth day after oviposition. In some sectioned host larvae the blastula 
stage of the parasites is found at the end of the sixth day, but this appar¬ 
ent precocious development occurs only in host larvae which were about 
ready to hatch from the host egg at the time of oviposition by the 
parasite. 
FIRST AND SECOND CLEAVAGE 
The first cleavage of the segmentation or embryonic nucleus takes 
place at about the second day (PI. 3, B). On the third day cleavage is 
found completed and two embryonic nuclei are visible in the embryonic 
region (PI. 3, C). Each embryonic nucleus measures 3.8 /x in diameter, 
or about the same size as the original segmentation nucleus. The two 
paranuclear masses are larger; one of the masses is seen migrating from 
the anterior end to that part of the trophamnion which surrounds the 
embryonic region. The entire parasite body has increased slightly in 
size during the second day, and now measures about 19.2 n in length 
and 9.1 \x in width. 
The second cleavage of the embryonic nuclei takes place between the 
fourth and fifth days and produces four nuclei in the embryonic region 
(PI. 3, D). Meanwhile the embryonic region has become spherical, and 
has increased in diameter to 10 jx. One of the paranuclear masses is 
now found beside the embryonic region. 
