Yolk utilization inScyliorhinus canicula,an oviparous dogfish

Synopsis Using plastic embedding techniques and semithin sections, in order to overcome the difficult sectioning of yolky eggs, we have been able to carry out histological study of the external yolksac from fertilization until birth in the oviparous dogfishScyliorhinus canicula. The endoderm and its contacting giant yolk nuclei remained very flat, seemingly inactive, during the larger part of development. They became activated only when the external yolksac (EYS) began to shrink. This activation increased along a vegetal-animal gradient in the EYS, but it was essentially restricted to the parts located near the yolk stalk. A statistical study of oocyte, yolk, embryo and newborn fresh and dry weights confirmed that the mass of dry tissue in the embryo (30mg) and in EYS wall (<1 mg) at mid-development were still very low compared to 0.8–1.5 g mass of yolk available for development. This explains why yolk weight remained practically the same during the first half of development. The end of this first period was marked by entry into the pre-hatching state at 85–115 days under laboratory conditions (14–16°C). At this time, yolk began to enter the spiral gut, where it was digested during the second half of development and during one week period after eclosion. Eclosion occurred 170–220 days after egg laying or extraction from oviduct. Two internal storage organs were studied biometrically in the newborn: the internal yolksac (IYS), and the liver, which was fully developed at birth. Both IYS and liver dry weights corresponded to about 10% of the original yolk, while the gut was only 2%, and the rest of the newborn body 58%. Thus, about 20% of yolk dry mass was consumed during development, a figure that is low for oviparous animals.