On the true metabolic rate of the chick embryo and the respiration of its membranes

Abstract

The variations in respiratory intensity of the extra-embryonic membranes in the developing hen''s egg have been investigated by manometric technique. The yolk-sac maintains a steady level of respiratory rate at 320 cmm./gm./hr. from the 3rd to the 15th day of development, after which it falls off, and suffers a diminution of about 40% by the time of hatching. That of the allantois begins at a level of less than 100 cmm./ gm./hr. on the 6th day, rises steadily to reach a maximum of 390 cmm./gm./hr. on the 13th day, and subsequently falls to about 250 cmm./gm./hr. by the time of hatching. These points of inflection are synchronous with the changes occurring in the growth of the membranes: the yolk-sac grows until the 16th day, after which it decreases in weight; between the 5th and 11th days the allantois grows more rapidly than any other constituent of the living system, but then remains at approximately constant weight. The percentage participation of the extra-embryonic membranes in the respiration of the intact egg has been found to decrease in a sigmoid curve from 32% at the 4th day to 4% at the 19th. But the actual amounts of O2 utilized by the membranes naturally follow their growth, and their summed respiration shows a maximum on the 14th day. By subtracting the O2-uptake of the membranes from the O2-uptake of the intact egg (identical breed and flock), a more true and precise measure of metabolic rate of the chick embryo can be obtained than that which neglects the respiration of the membranes. This is 1200 c.mm./ gm./hr. on the 6th day, falling to 800 c.mm/gm./hr. on the 19th. Since a maximum metabolic rate must exist during the development of the embryo this must occur some time before the 6th day. The Plastic Efficiency Coefficient of the developing hen''s egg has been recalculated in the light of this research. On the 6th day some 75% of the material absorbed by the embryo is transmuted into living cells, by the 9th day this figure has dropped to 66% but by the end of development it has risen again to 78%. The efficiency of the membranes is much higher than this; thus, on the 7th day no less than 95% of the material absorbed by the yolk-sac is turned into live weight, the figure has dropped to 72% by the 14th day, after which it rapidly becomes nil owing to the cessation of growth[long dash]but not of respiration[long dash]of the membranes. The day by day efficiency of embryo and membranes together is the sum of these 2 sets of data. In the absence of information concerning the calorific value of the membranes themselves and of the material being burned by them, the Apparent Energetic Efficiency could not be calculated so completely, but the already existing curve for the embryo has been corrected on the assumption that at any given moment the solids burned by membranes and embryo have an equivalent content of chemical energy.