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

Abstract

It has always hitherto been supposed, by those who have occupied themselves with the examination of the various metabolic aspects of the developing chick embryo, that the metabolism of the embryonic membranes might be regarded as negligible. Thus it has been the classical practice (as by Bohr and Hasselbalch (1900), Atwood and Weakley (1924), Lamson and Edmond (1914), Hanan (1928), and Murray (1925, b) ) to relate the amounts of oxygen entering, or of carbon dioxide leaving, the whole egg, to the wet or dry weight of the embryo, and thus to calculate the metabolic rate, leaving the embryonic membranes out of account. But it is clear that this can only be a first approximation, for the yolk-sac with its layers of endoderm and its vascular splanch-nopleure must possess an active metabolism of its own. The work of absorpation which it performs, as illustrated, for instance, in the recent histological description of Remotti (1927), can hardly be carried out with no expenditure of energy. The allantois, again, is covered with vascular splanchnopleure, and must be credited with some respiration of its own. But upon precisely what this respiration amounts to relatively to that of the embryo, will depend the true values for the metabolic rate of the embryo itself. It may be said, of course, that the extra-embryonic membranes consist of tissue which is physiologically just as young as that of the embryo could easily be calculated from their weights, assuming their respiratory intensities to be equivalent. But to argue thus would be to make a very large assumption, an assumption which, as will later be evident from the results reported in this paper, has no foundation in fact.