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This study examines the metabolic fate of total and individual yolk fatty acids (FA) during the embryonic development of the king penguin, a seabird characterized by prolonged incubation (53 days) and hatching (3 days) periods, and a high n-3/n-6 polyunsaturated FA ratio in the egg. Of the approximately 15 g of total FA initially present in the egg lipid, 87% was transferred to the embryo by the time of hatching, the remaining 13% being present in the internalized yolk sac of the chick. During the whole incubation, 83% of the transferred FA was oxidized for energy, with only 17% incorporated into embryo lipids. Prehatching (days 0-49), the fat stores (triacylglycerol) accounted for 58% of the total FA incorporated into embryo lipid. During hatching (days 49-53), 40% of the FA of the fat stores was mobilized, the mobilization of individual FA being nonselective. At hatch, 53% of the arachidonic acid (20:4n-6) of the initial yolk had been incorporated into embryo lipid compared with only 15% of the total FA and 17-24% of the various n-3 polyunsaturated FA. Similarly, only 32% of the yolk's initial content of 20:4n-6 was oxidized for energy during development compared with 72% of the total FA and 58-66% of the n-3 polyunsaturated FA. The high partitioning of yolk FA toward oxidization and the intense mobilization of fat store FA during hatching most likely reflect the high energy cost of the long incubation and hatching periods of the king penguin. The preferential partitioning of 20:4n-6 into the structural lipid of the embryo in the face of its low content in the yolk may reflect the important roles of this FA in tissue function.

The avian embryo is uniquely amenable to experimental manipulation. The most widely used models are chimeras resulting from heterotopic or orthotopic exchanges of rudiments between chick and quail embryos, according to Le Douarin's technique (1969). Cell migrations and fates are traced in these chimeras either through the identification of quail cell nuclei stained for DNA or by means of monoclonal antibodies that recognize a particular lineage in only one of the two species. The ontogeny of the hemopoietic and endothelial lineages, as enlightened through appropriately designed chimeras, is reviewed in the present article. Homologies recently disclosed in mouse and human embryo are emphasized. Finally, the possibilities afforded by retroviral somatic transgenesis in the avian embryo will be envisaged.

Measured boundary-layer conductance to heat exchange for bird eggs varies with egg mass to the 0.53 power. Calculations based on the Nusselt-Reynolds relationship for a sphere and the thermal properties of air indicate that the conductance of the boundary layer to heat and to mass at any wind speed other than still air should scale with mass to the 0.53 power. Although the boundary layer contributes little to the total barrier to mass flux between bird eggs and their environment, we show that it is the major barrier to the exchange of heat. From these observations we infer that birds incubating eggs in natural nests can alter only the gradient affecting mass flux between their eggs and the environment while having the capability to change both the gradient and conductance affecting heat flux.

Energy within an avian egg is used primarily to support the metabolic costs of growth and maintenance, deposited in embryonic tissues, or retained in residual yolk. In altricial species, embryonic growth rate and metabolic rate increase continuously during incubation, whereas in precocial species, embryonic growth rate declines shortly before hatching so that metabolic rate usually reaches a plateau before hatching. These differences mean that when altricial and precocial birds are compared using egg mass as a scaling variable, altricial species have lower costs of development, smaller hatchlings, and shorter incubation periods. Their eggs also have lower energy densities, however, and when egg energy content is used as the scaling variable altricial and precocial species cannot be distinguished except by egg size and energy density. This suggests that before hatching all avian embryos expand about the same percentage of the energy stored in their eggs. Altricial and precocial species do not appear to differ in energy cost per gram of yolk-free, dry hatchling produced (15.4 kJ/gm). The major difference between altriciality and precocity lies in the timing of hatching during the developmental process (earlier for altricial species than for precocial species) and in the amount of energy the parent originally deposits in the egg. We suggest that the first step in the evolution of altricial bird species from precocial ancestors was hatching early. This shortened the incubation period and reduced the amount of energy needed prior to hatching. Consequently, the female could reduce her energy investment per egg by decreasing size and increasing water content of the egg.

An experimental insecticide Wofatox 50 EC, injected into the embryonated eggs from hens and pheasants at 12th day of incubation, caused a significantly diminished body mass, a high incidence of developmental malformations and embryonic mortalities at higher dose-levels. The lower, in plant protection practice used concentration was no teratogenic or lethal on embryos.

The full endocrine system is developed during the embryonic stages of the domestic fowl and probably in all avian species. This involves the genesis and differentiation of endocrine cells and progressive acquisition of interactions between endocrine tissues leading to integrated series of control systems. The embryonic endocrine organs produce hormones with a variety of physiological roles (including homeostatic, homeorhetic, and anticipatory). The present chapter provides an overview of the recent advances in the endocrinology of the avian embryo. Particular attention is focused on the general phenomenon of endocrine development and on the ontogeny of the hypothalamo-pituitary (growth hormone)-somatomedin-growth axis, the hypothalamo-pituitary-thyroid axis, and the hormones of calcium homeostasis.

The relationships between weight (W) and age (t) of embryos of 27 species of birds are described by parabolic equations of the form W = a*(t-i)b*, where a*,b*, and i are constants and i accounts for a time lag before the onset of parabolic growth. A simple transformation gives values of a and b for the equation dW/dt = aWb. Some mathematical properties of the parabolic equation, relating growth rate (dW/dt) to weight and age, extrapolating different curves to their intersection (point of divergence), and relating the length of the incubation period to constants of the parabolic equation and to hatching weight, are described. Fitted constants for most species formed a cluster in the vicinity of a = 0.4-0.5 and b = 0.65-0.75, within which one could not differentiate precocial and altricial species, nor among precocial families. Pelagic seabirds consistently had reduced values of b and were the only group with i greater than 0 (5-19 days). The relation between fitted constants and incubation periods is discussed. The rate at which embryo growth rate decreases with respect to size (b) is directly related to the rate of accumulation of dry matter in the tissues, suggesting that both express an underlying program of development. This study emphasizes the relationship between growth rate and development of the embryo, illustrates the degree of evolutionary flexibility and diversification within and among taxonomic groups, and reveals the sensitivity of the very young embryo to environmental conditions.

At the start of incubation, heat flows through the egg by conduction only. During incubation, the embryo's developing circulation should augment conduction in the egg, distributing heat from the brood patch more uniformly through the egg. The augmentation of heat flow through the egg should be very small in small eggs, but should be larger in large eggs.

Chicken fetal antigen (CFA), a membrane antigen present on fetal chicken red blood cells is lost with chicken development, and reappears on the red blood cells of leukemic chickens. Seven avian species were found to possess CFA. A species hierarchy comparing the quantitative expression of CFA has been established. The levels of CFA expression with development are compared in the chicken and Japanese quail. Specific adsorptions of R-anti-CFA with avian red blood cells revealed the existence of multiple CFAs. Four groups of antigenic determinants (CFA a,b,c,d) have been characterized and defined by their expression among avian species. Multiple CFA determinants are discussed with regard to possible membrane alterations and gene function.

A new model of avian embryonic metabolism is proposed that more accurately describes the available data. The model assumes that embryonic metabolism can be described by the equation VO2 = A X massB + C X growth rate. Empirically determined values of A, B, and C were used to predict the values of these parameters for the species whose preinternal pipping rate of O2 consumption (PIP) and total energy used during incubation (TOT) have been measured. Different patterns of growth were assumed for altricial and precocial species. Predicted values of PIP and TOT are highly correlated with observed values (r2 = 0.966 and 0.979, respectively). The model predicts the pattern of metabolism in precocial species will vary with hatchling mass and incubation period; TOT will be greater in eggs with longer incubation periods and greater in precocial species than in altricial species because 70-80% of TOT is devoted to maintenance; PIP will be relatively insensitive to interspecific differences in incubation period, because the 80% of PIP devoted to maintenance is insensitive to incubation period; the energy cost of biosynthesis will average only one fifth of the energy content of the tissues produced; and the maintenance metabolic rate of an avian embryo is more like that of an adult reptile than that of an adult bird. Shell conductance may not be exactly inversely proportional to incubation period, because it represents a compromise between the requirements for limiting water loss and permitting adequate exchange of respiratory gases. The model overestimates PIP and TOT in species with very long incubation periods; these embryos may have unusually low metabolic intensities.