Electron microscopic observations on the yolk sac of the spiny dogfish, Squalus acanthias

Abstract

Yolk sacs from pregnant spiny dogfish, which were collected throughout one summer, were categorized according to functional stage. Those in the middle of the first summer of pregnancy were observed to function both in nutritive and in respiratory transport. Yolk sacs from late first-summer pregnancies appeared to be engaged in nutritive transport alone. In the second summer of pregnancy, the membranes apparently performed neither nutritive nor respiratory transport. Fine structural changes in both respiratory and nutritive “barriers” as the transport function of each was lost were noted and are described. Digestion of yolk has been observed to begin within a syncytial cytoplasm which closely invests, and is continuous with, the yolk mass. This cytoplasm has been termed “vitelloplasm.” Both the digestion of yolk within the vitelloplasm and the transport of the digested product across the various layers of the “yolk—blood barrier” appeared to be mediated by Golgi membranes.