May 24,1924 Polyembryonic Development of Platygaster vernalis 
833 
cease development and become aborted. The egg of Platygaster vernalis is not 
encysted by host tissue, but is able to continue its development by reason of its 
location in the chyle of the host’s stomach. The chyle, it should be recalled, is 
in reality the sap of the wheat plant, although it may be altered chemically by 
the nuclei of the intestinal lining; so that in one sense the parasite body may be 
regarded as developing in host tissue. 
DEVELOPMENT OF THE PARASITE BODY IN THE HOST LARVA 
The development of the embryos of Platygaster vernalis covers a period of 
about thirty days and takes place largely during the month of May, while the' 
host larva is feeding and maturing upon the wheat plant. The presence of the' 
parasite body in the stomach of the host apparently does not affect its growth. 
Like other species of polyembryonic parasites the P. vernalis larvae are not de¬ 
veloped to the point where they begin feeding upon the host until after the host 
is completely grown. Precocious development of the parasite larvae, and feeding 
before the host is fully grown, would result either in their starvation or a reduc¬ 
tion in the number produced from a single egg caused by the feeding of some of 
the parasites upon others of the brood. 
The pupal and adult stages of the parasites are developed in individual cocoons 
(PI. 1, E) formed by the larvae in the carcass of the host during late summer. 
The adults remain in the cocoons during the winter and emerge the following 
spring. 
EARLY CLEAVAGE AND THE FORMATION OF THE POLYGERM 
About the second day after oviposition the cleavage nucleus migrates toward 
the center of the egg, where it divides to form two daughter embryonic nuclei 
(PI. 3, C) of equal size. A second, third, and fourth cleavage of the embryonic 
nuclei take place between the third and sixth days after oviposition. These divi¬ 
sions of the nuclei produce parasite bodies which contain four, eight, and sixteen 
embryonic nuclei respectively (PI. 3, D-F; PI. 4, A). The embryonic nuclei 
are located in the central part of the parasite body in an enlarged embryonic- 
region, which was differentiated around the cleavage nucleus when the parasite 
body was one day old. They are all of about the same size and measure approxi¬ 
mately 4 /z in diameter. 
While the first four cleavages take place, the trophamnion becomes elaborated 
proportionately. The earliest change is observed at the first cleavage, when one 
of the paranuclear masses divides to form two masses which immediately proceed 
to increase in size (PI. 3, C). At this time the parasite body measures 24.3 y in 
length and 10 y in width. At the. second cleavage four paranuclear masses (PI. 3, 
D) are observed in the trophamnion. At the third cleavage there may be as 
many as twelve paranuclear masses. As is shown in Plate 3, E, the masses are 
distributed uniformly throughout the trophamnion. The parasite body now 
measures approximately 27.3 y in length and 14.5 /x in width. Its form is still 
ovoid, and similar to that of the original egg. 
The development of Platygaster vernalis reaches that stage which is comparable- 
to the polygerm stage of other polyembryonic hymenopters at the end of the- 
fourth cleavage (PI. 4, A), at which time the parasite body contains sixteen nuclei 
in the embryonic region if all nuclei have divided regularly. Each embryonic 
nucleus then becomes separated from the others, and, surrounded by a bit of the 
embryonic cytoplasm, the cytoplasm with its nucleus becomes invested by a 
membrane. A cell is thus formed and recognized as a germ. The germ becomes* 
the progenitor of one or two parasites, depending upon whether or not it divides* 
once in the morula stage. The parasite body now represents a typical polygerim 
