ABSTRACT
Yolk-sphere formation in the hen’s oöcyte has been studied by electron microscopy using the following stages: Balbiani body present, Balbiani body dispersed, large vesicles visible, well-defined yolk present. A detailed description is given of each stage.
Raw materials from which the yolk is formed appear to enter the oöcyte in two ways. The first is probably by pinocytosis, the second by specialized pieces of follicle cell apparently becoming engulfed by the oöcyte. These pieces of follicle cell have been described in detail elsewhere under the name of ‘lining bodies’.
Prior to yolk formation the mitochondria and Golgi bodies migrate to the periphery of the oöcyte, the numbers of free lipid drops in the cytoplasm become reduced and two new structures appear in the oöcyte. These are respectively electron-dense particles measuring 300 Å in diameter and specialized structures that may be termed ‘yolk spindles ‘. The latter are about 700 Å in T.S. and contain within them a rod that is about 50 Å in diameter.
The yolk spheres develop from large, membrane-bounded vesicles that appear in the ooplasm. There is no direct evidence as to the origin of these vesicles though several alternative suggestions are made.
Yolk spindles cluster around some of the large vesicles and appear to become incorporated into them. It is suggested that the yolk spindles influence the large vesicles to transform into primary yolk spheres, though the mechanism is not understood.
As the large vesicles become recognizable as primordial yolk spheres, they acquire osmiophilic subdroplets as well as more granular material. Frequently they also contain many 300 Å dense particles, as well as degenerating lining bodies. The mechanism by which these materials enter the primordial yolk spheres is unknown.
Each yolk sphere within the oöcyte is surrounded by a well-defined unit membrane. By the time the egg is laid, however, it has become difficult to demonstrate this membrane. It is suggested in this paper that the unit membrane of the yolk sphere retains its normal appearance so long as it remains in contact with living cytoplasmic structures. This contact is retained only until the oöcyte is about 2·5 mm in diameter, for at about that time the nucleus and cytoplasmic components stream up to the animal pole where they remain.
