1. The growth-curve of the embryo of Salmo fario is asymmetrical and inflects when the embryo has completed approximately 70 per cent, of its development.

2. For any given temperature of incubation the final size of the embryo is proportional to the total amount of yolk present in the newly fertilised egg. If the amount of yolk is reduced experimentally the final size of the embryo is reduced, but its morphological form is normal.

3. The growth-rate of the embryo is shown to be proportional to its own size and to the amount of yolk in the yolk sac. Assuming that the amount of yolk required for the maintenance of a gram of embryo at all periods of its life is constant, the dry weight of the embryo (x) can be expressed in terms of the yolk in the yolk sac (y) by means of the equation

x = y0 - y - k2 logey0 + k2/y + k2,

where y0 = the dry weight of yolk in the newly fertilised egg, and where k2 is a constant which alters with the temperature of incubation.

4. On theoretical grounds there should be a period towards the end of incubation when the total wet weight of the larva is decreasing, whereas the wet weight of the embryo is still increasing. This is shown to be the case.

5. If the temperature is raised, the final size of the embryo at the end of incubation is reduced. This fact is also deducible from the initial assumption that the specific rate of growth is proportional to the amount of yolk available.

6. The factors controlling the specific rate of growth of a fish embryo are, therefore, (i) temperature, (ii) food, i.e. the amount of yolk in the yolk sac. These two factors operate in precisely the same way as on a culture of bacteria, and it may be concluded that the processes of metazoon growth in vivo are similar to those of bacteria.

7. The so-called "efficiency" of development falls as incubation proceeds, and no single figure holds good over more than very limited periods.

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