The Energy Sources in Ontogenesis

ABSTRACT By studying the respiratory quotients of developing embryos, valuable indications can be obtained as to the nature of the substances undergoing metabolic oxidation within them. Many different embryos have been made the subject of such studies, but probably only the most recent data can be wholly accepted. The values given by the earlier workers were for the most part calculated by dividing the amount of CO2 given off from a sample of the material by the amount of oxygen consumed under the same conditions. But such a method of calculation is liable to be entirely misleading, since the amount of CO2 given off from the material may be very different from the amount actually produced by metabolic oxidation. Thus, as Richardson (1929) writes: “Even when the apparatus is technically correct, errors of interpretation may arise. The quotient as measured may not reflect the concurrent oxidation. In this event, it is convenient to designate the observed quotient as the measured quotient, and the supposed quotient due to oxidation as the metabolic quotient.” It is clear that if CO2 be retained in the material under examination, e.g. on account of pH changes or the deposition of calcareous matter in the developing embryo, the measured quotient will be lower than the metabolic quotient, unless the amount of bound CO2 can be determined before and after the period during which the oxygen uptake is measured. These considerations would apply particularly to the snail eggs which formed the material for the present study ; here, as we shall see, large amounts of CO2 are required for shell formation, and consequently any quotients obtained by the earlier methods would have been grossly inaccurate and quite useless as the basis of any hypothesis as to the energy sources of the developmental processes. It is only of recent years, with the development of manometric technique, that it has been possible to make proper allowances for any retention of CO2, a fact which must be borne in mind in considering the earlier data.