When the egg of Alaska pollack, Theragra chalcogramma attains to actual maturity, it decreases in specific gravity and floats in the sea water, whereas the specific gravity of immature egg is smaller than that of the ordinary sea water. Consequently, it may be admissible to consider that the eggs which sink in sea water immediately after discharge are not yet fully matured, i. e. that they are immatured eggs having no capacity for normal fertilization. Fertilized or developing eggs float at least till the hatching stage, because, upon fertization, they have gained a resistance against the hypertonicity of the surrounding sea water. The unfertilized niature egg is parthenogenetically activated without insemi-nation when it comes in contact with sea water, undergoing the same changes as the fertilized though formed blastodisc does not begin to be divided, and it gains a resistance against the hypertonicity of the surrounding medium ; accordingly it can remein for some time floating in the activated state in sea water. But such an egg begins to sink afterwards from loss of resistance to hypertonicity because of the lowering of its vitality. That is, the diffusing out of water from egg interior occurs and increase of specific gravity of the egg results. Egg-activation and development are much affected by the salt concentration of the medium and it may be considered that isotonic and also hypotonic sea water are unsuitable media for activation and development of the egg.