Ultrastructure of the secondary yolk sac in pig's embryo.

During the investigated period of development the pig embryo secondary yolk sac consists of two parallel epithelia (endodermal and mesodermal) separated by a layer of vascularized mesenchyme. Endoderm cells and blood vessels undergo morphological changes connected with the embryo age. The development of the blood vessels is at the vasculogenesis stage. Blood stem cells, erythroblasts, megakaryocytes and platelets were observed in the lumen of vessels. A comparison our results with the development of the pig embryo liver shows that the transferring the haemopoetic function from the yolk sac to the liver begins on about 27 dpc. Until 51 day of embryo development the yolk sac does not show any signs of involution. Possibly yolk sac supports the liver in its haemopoetic function until the time when that organ becomes a fully efficient haemopoetic organ.