DEVELOPMENTAL BIOLOGY 33, 62-79 (1973) The Composition of Fetal Fluids of the Marsupial Macropus eugenii MARILYN B. RENFREE’ Department of Zoology, Australian National University, Canberra, A.C.T. 2600, Australia Accepted January 17, 1973 The composition of the fetal fluids of the marsupial, Mucropus eugenii, and their relationship to maternal serum and uterine secretion is described. The results show that growth of the embryo can be divided into two phases. In the first phase, when the vesicle is not attached to the uterus, the composition of the vesicle fluid resembles maternal serum only in its glucose and urea content, and is more concentrated in free amino acids and more dilute in proteins. The electrophoretic pattern resembles that of uterine flushings. The second phase begins at attachment of the yolk sac to the uterine epithelium when the vascular region of the yolk sac has developed and embryogenesis begins. At this time, the composition of the yolk sac fluid is notably changed, particularly in its protein components, although maternal serum and endometrial secretion do not change. Most of the proteins found in the yolk sac fluid during this phase are probably of fetal origin. Each fluid compartment has its own unique composition; the allantois has high concentrations of urea, the amnion is dilute in all components, and the yolk sac has high concentrations of amino acids at all stages of gestation and of glucose during late gestation. The vascular area of the yolk sac is probably the important region for respiratory exchanges, while the nonvascular region persists as the route of transfer of nutrient material. It is suggested that the second phase is one of increasing fetal autonomy, made possible by the differentiation of organ systems (some of which appear to be functional early), as well as by possible conversion of material in the yolk sac itself. INTRODUCTION In mammals, all the embryonic sacs contain fluids which vary in composition. The components are not freely exchanged either between the compartments or with the uterus, and therefore reflect the several activities of placental transport and fetal metabolism. In the rabbit, for example, immunoglobulins cross the yolk sac pla- centa but not the allantoic placenta (Brambell, 1970), but the proteins of the exocoelomic, allantoic, and amniotic fluids, while resembling each other, differ widely from fetal and maternal sera (Brambell et al., 1953; Wild, 1965, 1966). Even before implantation, protein patterns of blastocyst fluid differ greatly from serum and endometrial secretion, and progres- sively increase in number and concentra- tion as gestation proceeds (Beier, 1968; Hamana and Hafez, 1970) possibly as the result of conversion from the amino acids ‘Present address: Department of Zoology, The University of Tennessee, Knoxville, Tennessee 37916. 62 Copyright 0 1973 by Academic Press, Inc. All rights of reproduction in any form reserved. which are found in high levels in the fluid (Lesinski et al., 1967). Similarly, ionic content, glucose, and other substances all change in concentration relative to serum and endometrial fluids before and after implantation (Lutwak-Mann et al., 1960; Jacobsen and Lutwak-Mann, 1956; Kodi- cek and Lutwak-Mann, 1957; Lutwak- Mann, 1962). These general trends have been found in other mammals such as the rhesus monkey (Bangham et al., 1961; Kerr and Kennan, 1969), the rat (Wirtschafter and Williams, 1957a,b) and the human (Bosnes, 1966). However, in these species either the yolk sac, allantois, or exocoel is lost or emptied during gestation, and the chorioallantoic placenta becomes the main route for transport of essential materials (Brambell, 1970). Further, the intimacy of contact does not oblige the fetus to be autonomous in its functions since it is able to rely entirely on the placenta. In marsupials the embryonic compart- ments retain their integrity throughout