836 
Journal of Agricultural Research 
Vol. XXVIII, No. 8 
host, the parasite body being tossed about within it by peristaltic action, at least 
until the poly germ stage is formed. Thereafter, the polygerm becomes, as a 
rule, lodged in one end of the mid-intestine, where the germs develop into true 
embryos, and finally into the primary larval stage before the parasite body is 
broken up. 
The development of Platygaster vernalis is therefore similar, in a general way, 
to that of Polygnotus minutus. In other described polyembryonic species, such 
as Copidosoma gelechiae and C. truncatellum , development takes place in the 
body cavity of the host, the polygerm breaking up into secondary masses which 
are held together in a group by adipose tissue of the host in the case of C. gele¬ 
chiae , and scattered throughout the body of the host in the case of C. truncatellum. 
Each individual blastula or embryo of C. gelechiae and C. truncatellum becomes 
separated from all others and is invested with a portion of the trophamnion and 
paranucleus, thus completing its development independently of any other indi¬ 
vidual. In Platygaster vernalis and Polygnotus minutus the parasites are de¬ 
veloped to the primary larval stage in the common original trophamnion, al¬ 
though, as mentioned above, the parasite body may, in a few instances, at least 
in P. vernalis , become accidentally divided into two or three secondary masses. 
In this event one of these secondary masses may not develop completely. 
Before cleavage the egg contains a cleavage or embryonic nucleus in a differ¬ 
entiated embryonic region, the remainder of the egg comprising the trophamnion, 
which contains two paranuclear masses of polar body origin. Four cleavages of 
the original embryonic nucleus result in the production of twelve to sixteen 
daughter embryonic nuclei, each of which lies within a small portion of the em¬ 
bryonic plasm and forms a germ. Some of the germs when composed of eight 
embryonic nuclei divide to form two daughter germs, but in any event each 
normal healthy germ finally develops into an embryo. From eight to twelve 
embryos are thus developed from a single Platygaster vernalis egg. 
The development of the Platygaster vernalis egg therefore represents a simple 
type of polyembryony; not as simple as that shown by the writers (4) in P. 
hiemalis , nor as complex as that demonstrated in other polyembryonic insects 
by Marchal ( 5 ), Silvestri (7), and Leiby (3). In P. hiemalis we have shown that 
immediately after second cleavage some of the eggs divided into two equal parts, 
each part containing two embryonic nuclei and twfc> paranuclear masses which 
together form an embryo. Twin embryos are thus developed from some eggs, 
while other eggs do not become so organized, and develop but a single individual. 
In P. vernalis approximately four cleavages take place before the germs are 
formed and an average of eight embryos are thus matured from a single egg. 
In Copidosoma gelechiae the senior writer (3) has shown that the germs are not 
organized until after the seventh cleavage, with the result that from 150 to 225 
embryos are produced from a single egg. In any event, it appears that the num¬ 
ber of embryos produced nearly always approximates the number of parasites 
that the host larva is able to mature. The size of the host appears to be the 
governing factor. Where but one egg is normally deposited in the host egg, 
as in the case of C. gelechiae and P. vernalis , cleavage continues to the point 
where the maximum number of parasites is developed that the host can mature. 
When cleavage extends beyond that number the embryos become aborted in 
their different stages. In the case of P. hiemalis two individuals develop from 
some of its eggs, and a single individual from others. This parasite therefore 
deposits from five to eight eggs at a time in one host egg, and in this way are 
developed the maximum number of individuals of this species that the Hessian 
fly larva can mature. 
The failure of some of the germs to keep pace in their development with other 
germs in the same host has been briefly referred to above; the fault being ascribed 
