EMBRYONIC MEMBRANES 359 



medium — either the blood or gut contents of the host — and, as 

 will be mentioned later, a certain amount of the fluid is absorbed 

 to nourish the developing embryos. As development proceeds 

 and the embryonic area becomes enclosed in a cavity within the 

 trophamnion, fluid accumulates in the narrow intervening space 

 (Fig. 90, C). It is possible that, owing either to the osmotic action 

 of this fluid or to pressure exerted on it during growth of the egg, 

 the embryonic mass becomes constricted with the result that it 

 finally becomes separated into two groups of blastomeres, each 

 capable of development into an individual parasite. The simple 

 condition described in Platygaster kiemalis appears to be explain- 

 able on this hypothesis and that the more complex cases of 

 polyembryony are highly specialised phases derived from it. 

 Marchal lays stress upon the influence of relatively sudden changes 

 of osmotic pressure in promoting embryonic dissociation, and 

 points out that a parallel is afforded by the experimental 

 results obtained by Loeb and by de Bataillon with the eggs of 

 sea urchins and of the lamprey. By using fluids exerting 

 different degrees of osmotic pressure, they were able to induce 

 separation of the blastomeres and their development into distinct 

 individuals. 



The Embryonic Membranes. In recent years many instances 

 have been brought to notice, among endoparasitic Hymenoptera, 

 in which the embryo is surrounded by a cellular membrane. Its 

 existence has been observed in members of all the main parasitic 

 groups, but in some cases only in its last stage as a remnant still 

 attached to the newly hatched larva. Authors have variously 

 referred to it as the trophic membrane, trophamnion, amnion, 

 serosa or pseudoserosa. Its method of origin, however, has only 

 been determined in a few instances, and there appears to be little 

 doubt that it is not the same in all cases. The fact that an 

 embryonic membrane of this type is developed in eggs very 

 deficient in yolk, or almost devoid of it, has led to the conclusion 

 that it plays an important part in nourishing the developing 

 embryo. It is believed to absorb and elaborate food material 

 from the host tissues and act as a means of transferring such 

 products to the embryo with which it is in close association, 



