Before entering upon any discussion or comment on the above description of Polyembryony, a summary of the main facts known will be given.
(1) Polyembryony in the Hymenoptera parasitica is process whereby the single egg, instead of producing a single embryo, often produces several score or more.
(2) The polyembryonic Hymenoptera are generally small insects about 1 mm. in length.
(3) The polyembryonic parasite lays from one to ten or more eggs in the ovum of the host.
(4) This oviposition does not kill the host eggs. Larvæ hatch from the latter in the normal way, but contain the eggs of the parasite, generally in the hæmocoæl cavity.
(5) The parasite's egg gives off polar bodies, and may or may not be fertilised in the normal way.
(6) The polar bodies rest for a time, but then break into activity, forming an actively growing mass, or collection, of nuclei.
(7) That part of the egg cytoplasm containing the segmentation nucleus separates off from the outer part containing the active polar nuclei, and the germ-cell determinant goes to the former, but later becomes absorbed and lost to sight.
(8) The polar cytoplasm or ooplasm containing the polar nuclei forms an investing sheath around the contained embryonic ooplasm, which later gives rise to the embryos. The polar ooplasm nourishes the inner embryonic mass and acts as an amnion or placenta. Hence the name trophoamnion.
(9) The nuclei of the tropho-amnion derived from the original polar body nuclei become very numerous by division, and the tropho-amniotic cytoplasm becomes very granular in the region of the nuclei.
(10) Certain cells of the embryo, either hæmolymph or fat cells or both, form an outer covering to the parasitic germinal mass. This host-covering later becomes much stretched and epithelial in character. In some forms it is not well developed.
(11) The primary embryonic cell separated off at the time when the polar nuclei begin to become active, has already divided many times to give rise to many germinal masses. The parasitic body lying inside the host hæmocoel. may now be called a polygerm.
(12) The polygerminal embryonic masses, keeping on dividing till as many as a hundred or more masses may be produced, later become constricted into areas each containing an embryonic mass surrounded by two membranes, the outer host-epithelial and the inner tropho-amniotic layer.
(13) The shape of the entire polygerminal mass differs in different forms. In some it is a ramifying cylindrical body, in others a shapeless mass, constricted here and there by the outer membranes.
(14) Bach separate germinal mass is now a spherical or ovoid morula containing a score or more cells. The latter keep dividing.
(15) The embryonic or germinal mass now begins to differentiate further; it loses its sub-spherical shape and becomes elongate, while dorsal, ventral, and lateral sides of the future embryo can be distinguished.
(16) The stomodæum and proctodæum are formed by invaginations of the two extreme ends of a ventral groove. The ectoderm is formed by a rearrangement of the outer cells of the morula. The endoderm and mesoderm are formed in situ by a modification of the more centrally-placed cells of the embryo.
(17) The larvæ at a later stage break away from their membranes and are free-living for a time. They later eat up nearly everything in the host-caterpillar's body and then pupate inside (in some cases, however, apparently outside) the body of their host.
In this review I have not so far given any data with regard to the sexes of parasites emerging from one caterpillar. Broods may be purely male or female, or mixed. In Patterson's cases 55 per cent, of all broods were female. Moreover, the average number of females emerging from a single carcase is 198 as compared with 175 for males. Some of the mixed broods doubtless arose from two or more eggs, fertilised and unfertilised; but Patterson makes the interesting suggestion that such mixed broods may also arise from a single fertilised egg by a process of disjunction of the sex chromosome during early cleavage stages (13). As far as is known fertilised eggs produce females, unfertilised males.