Aug. as. X933 Twinning and Monembryony in Platygaster hiemalis 343 
phamnion are two paranuclear masses that usually appear decidedly 
crescentic in shape (PI. 4, A, B). The paranuclear masses continue to 
increase in size through their absorption of elements from the host 
tissue which surrounds the entire germ or early blastula. 
THE ABORTION OF SOME EGGS 
* 
It has been shown above that on an average six to seven eggs are 
deposited by the female parasite in a host egg or newly hatched larva. 
If each of these developed twins successfully, the number of parasites 
produced would always be equal to twice the number of eggs deposited. 
Hence the average number of parasites reared from host puparia should 
be 12 or 14. It has been shown by the junior author (j), however, that, 
on the basis of the rearings of 100 puparia, the average number of cocoons 
produced in a single puparium is 6.52 and the average number of parasites 
reared is 6.31 per puparium. 
One of the reasons why the number of parasites actually reared does not 
more nearly approximate 12 or 14 is the failure of some of the eggs to de¬ 
velop beyond the maturation and fertilization stages. It is common to 
find eggs in a stationary or degenerate stage of development at a time 
when other eggs in the same host, deposited at the same time,® are about 
to twin or have already passed the twinning stage. Figures E, F, and G 
of Plate 3 represent three types of eggs or parasite bodies found in a 
single host that originally contained five parasite eggs. Two of these 
five eggs have reached the twinning stage (one shown at G on PI. 3); 
one egg has reached the stage in which it contains two paranuclear 
masses, and its embryonic region has not as yet divided (PI. 3, F); and 
the fourth and fifth eggs have not developed to the stage of first cleavage 
of the segmentation nucleus (one shown at E on PI. 3). Eggs of the 
latter type become aborted. They fail to develop, apparently because 
they did not become enveloped by host tissues from which they could 
receive the nutriment necessary to continue their development. At any 
rate, parasite bodies that are in the normal course of development at the 
cleavage or later stages are always observed surrounded by host tissue. 
Aborted or degenerating eggs nearly always stain less deeply than 
normal eggs; their nuclei are smaller and frequently appear irregular 
around the edges, while the nuclei of normal eggs are regularly rounded. 
Approximately one-third of the eggs deposited by the parasite at one 
time become aborted. 
monembryonic development of some eggs 
If twin germs were regularly produced from all nonaborted eggs, an 
even number of germs or blastulas should always be found in each host 
larva. That twinning is not the. only process of development in this 
species is indicated by the finding of an odd number of germs or blastulas 
in perfect serial sections of parasitized larvae. As examples, two sec¬ 
tioned hosts show seven germs each, a third shows seven blastulas, several 
show three blastulas, and others show either an odd or even number of 
both the germ and blastula stages of the parasite. 
Moreover, a further study of the stage of the parasites in a host con¬ 
taining an odd number of parasites shows that the parasite bodies 
8 Oviposition into the host was observed under the binocular microscope and in most of the instances 
by far, only one oviposition was permitted in each egg. 
