Abstract
Electron probe analysis of the embryonic chick chorioallantoic membrane reveals that calcium is located at discrete sites in the ectodermal cell layer of the membrane. The calcium loci are found at characteristic sites within distinctive "capillary-covering" cells. Evidence is presented that the calcium identified by electron probe analysis is that pool of calcium undergoing active transcellular transport. Mitochondria and/or endoplasmic reticulum do not appear to be responsible for the observed calcium sequestration. It is suggested that the calcium might be contained within endocytotic vesicles, and a hypothesis is advanced which could reconcile endocytotic transcellular transport by specialized cells with previously proposed models for intracellular calcium ion regulation.