a `` '' topping-up '' '' hypothesis is opposed whereby endodermal cells of the area vasculosa continually sequester extracellular yolk material , which is incorporated into a digesting intracellular yolk drop while , at the same time , digested yolk products are being transported across the vascular pole of the endoderm to the extraembryonic circulation and thence to the embryo .
C1183500
y endodermal cel
C0014144
endode