But in a few European and North American species, and in a great many inhabitants of the tropics, the egg is large and a considerable portion of it persists for a long time as a yolk-sac. Although the segmentation is always complete, it is very irregular in these types, some of which make a distinct approach to the meroblastic egg.
Expansion of the extra-embryonic coelom cavity allows the yolk sac and allantois to expand into the coelom cavity allows the yolk sac and allantois to expand into the coelom cavity from the gut of the embryo.
Does the human fetus temporarily develop gills, a tail, and a yolk sac?
Is yolk sac size in hatching fishes a good indicator of how long a fish can survive without food?
Often young birds acquire the infections from the hen or through navel or yolk sac infections.
