Inter-specific transfers of eggs between rat and mouse.

INTER-SPECIFIC transplantations of mammalian eggs offer one possible way of studying the significance of intrinsic and external factors in embryonic development and their mutual interactions. The variety of problems which can be attacked by this method is dependent, of course, on the length of time during which development can proceed with a given combination of donor and recipient. The development of an egg in a specifically foreign animal can be adversely affected from the very beginning, though this is more likely to occur from the time of implantation onwards when all morphological, physiological,, and immunological differences between the embryo and the mother come to action. The available information, though very scanty, supports the view that during early development up to the formation of the blastocyst, the egg is highly tolerant of a foreign environment. Cleavage and blastocyst formation of sheep eggs was observed in the rabbit (Averill, Adams, & Rowson, 1955) and the lack of any deleterious effect was proved by their normal development after re-transplantation to another sheep. Sheep and goat eggs, after reciprocal transfers, develop even further, as living sheep embryos aged 30, 40, and 45 days were reported by Warwick & Berry (1949), and the birth of a dead but already fully coated individual was described by Lopyrin, Loginova, & Karpov (1951). According to the former authors the opposite combination, i.e. goat eggs transferred to sheep, is less successful as the embryos die about the 22nd day of gestation. Briones & Beatty (1954) transferred eggs between mouse, rat, guinea-pig, and rabbit (in 9 out of the 12 possible combinations). These authors were interested only in finding whether development was possible at all. Accordingly, the experiments were terminated after 1, or sometimes 2, days and so did not cover the whole of pre-implantation development. Types of development recorded were cleavage, change from morula to blastocyst, change of form and size of