1. During the yolk sac phase of development the larval trout absorbs water from the environment. The percentage dry weight of the embryo itself remains constant at about 16 per cent. and that of the yolk at about 41 per cent.
2. The efficiency of development between 50th and 80th day of incubation is constant about .63 ; in other words in order to produce 63 gm. dry weight of embryo, 100 gm. dry weight of yolk are consumed. The loss of 37 per cent. dry weight of yolk is accounted for by the oxidative processes going on in the embryo.
3. Vertebrate animals which lay eggs on land must provide an additional water supply to that present in the yolk. The type of egg laid by reptiles or birds is most simply derived from some aquatic type which possessed a perivitelline space wide enough to allow development to proceed to an advanced stage before the act of hatching occurred.
4. Attention is drawn to the fact that the efficiency of development of various types of animal organisms is of the same order of magnitude.