Morphogenesis in the amphibian embryo fertilization and blastula formation

The morphology of the cortex of the amphibian egg is discussed, with special reference to the hyaline membrane.The physical properties of the cortex are reviewed, especially those evidenced by the wound healing reaction. The participation of the hyaline membrane in this process is particularly emphasized.An analysis of the activation processes normally associated with fertilization indicates that they may be accounted for by three cortical contractions, two radially symmetrical, with center at the animal pole, and one asymmetrical (or bilaterally symmetrical) originating at the point of sperm entrance. The contractions may be spatially correlated with three wounds normally afflicted on the cortex, viz., the expulsion of the polar bodies and penetration of the sperm. The first cortical contraction leads to separation between egg surface and vitelline membrane, and consequently to rotation of orientation, the second leads to formation of the perivitelline space, and the third to grey crescent formation.The mechanisms of blastula formation and of volume regulation are finally discussed. The strong adhesion between the cells and the hyaline membrane are of special importance in the former process. The hydrostatic pressure exerted through the cortex tension of the individual cells is suggested to account for the transport of water against the osmotic pressure difference from cells to blastocoele. The tension in the hyaline membrane developing in the late blastula and early gastrula is suggested to control the volume of the embryo.