 In the first phase, lasting two months to an embryonic length of , the embryo relies on yolk for sustenance and gas exchange occurs across its surface integument and possibly also the yolk sac.During the second phase, which also lasts for two months to an embryonic length of , the external gill filaments develop and the yolk sac begins to be resorbed, the embryo ingesting histotroph (a nutritious substance secreted by the mother) in the meantime.In the third phase, lasting six to eight months, the depleted yolk sac is converted into a placental connection through which the fetus receives nourishment until birth.