Avian embryonic development is influenced by oxygen availability during incubation. We propose to study ahe effects of 02 availability upon: 1) tissue capillarity, and 2) behavior and growth after hatching. In addition, 3) the mechanisms by which changes in 02 availability affect embryonic growth will be investigated. 1) Capillary density in brain, heart, skeletal muscle, and the chorioallantois will be measured by morphometric and volumetric technics during the course of incubation, using tissue from "growth-accelerated" (produced by 60% incubator 02 concentration) and "growth-retarded" (produced by coating part of the eggshell) as well as "contarol" (incubated in 21% 02) chicks. 2) Behavior will be studied using Pavlovian conditioning paradigms with heat as the unconditioned stimulus. Whole body growth will be determined by daily weighing of the chicks for one month after hatching. Growth of organs will be measured by analyses of wet and dry weights, and analyses for DNA and protein. 3) The mechanisms by which changes in 02 availability affect embryonic growth will be investigated by measuring the plasma concentration of anabolic hormones in experimental and control embryos, the plasma concentrations of glucose, free fatty acids, and amino acid nitrogen, and by morphologic examination of organs which are knwn to secret anabolic substances. The tissue concentrations of superoxide dismutase, lactate, pyruvate, NAD and NADH will be measured as indicators of tissue 02 tension. Tissue oxygen tension will be measured directly in cultured embryos. Tissue ATP, ADP and AMP concentrations will be measured in brain, hear, and skeletal muscle. We will attempt to correlate growth rate (judged by changes in weight, DNA and protein) and capillary density of each tissue (heart, brain skeletal muscle and chorioallantois) with tissue 02 tension or its indicators (superoxide dismutase concentration, lactate/pyruvate and NAD/NADH0 with the plasma concentrations of anabolic hormones and lipids, and with morphological evidence of secretory activity in the cells of the thyroid, pituitary and pancreatic islets.