ABSTRACT
In a series of recent papers Gray (1926–8) has shown that the growth-rate of the embryo of Salmo fario is proportional to its size and to the quantity of yolk present in the yolk sac. The yolk is used for two simultaneous processes—it is being converted into embryonic tissue and it is providing the material which is used up by the embryo in respiration ; there is, according to Gray, no measurable amount of yolk used for any other purpose. Under these circumstances the dry weight of the embryo at the conclusion of development must be equal to the dry weight of yolk in the newly fertilised egg, less the amount which is required for maintaining the respiration of the embryo. By incubating eggs at 16°, 10° and 5° C., Gray concluded that the higher the temperature of incubation the smaller was the final size of the embryo and the greater was the percentage of the original yolk consumed by the respiration of the embryo during development. This conclusion is in harmony with the observations of Graham-Smith (1920) who showed that the lower the temperature of incubation the greater is the maximum population of bacteria which can be obtained from a given volume of culture medium. The work of Bathélemy and Bonnet (1926) with the eggs of the frog are, however, at variance with these conclusions—at least to some extent. In one experiment performed by these authors the results conform to those obtained by Gray with the eggs of the trout, but in another experiment the temperature of incubation appeared to have no effect on the final size of the embryo.
As shown by Gray a certain amount of yolk remains unabsorbed at the time when all trace of an external yolk sac has disappeared. During this period the dry weight of the larva falls unless it receives external food. In the present paper, the total absence of yolk infers an absence of yolk which is visible on the ventral surface of the larva and not an absence of “internal” yolk.
AIn Gray’s paper (1928a), p. in, there is a misprint. The percentage dry (and not thewet) weight of the embryo is 16 per cent, and that of the yolk is 41 per cent. The correct figures are of course used by Gray in his calculations, and are clearly established by him in his earlier paper (1926).
