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They observed that interactions between the embryo and the vitelline membrane provided a source of mechanical force.

The human ovum is a flattened spherical cell, made up of a very delicate cell-wall, called the vitelline membrane; outside this is a comparatively thick membrane, the zona pellucida, which is properly not a part of the cell.

The eggs are small; the diameter of recently-deposited eggs is about 1.3 mm. and that of the vitelline membrane is about 1.5 mm.

The only membrane enclosing the individual eggs is the vitelline membrane.

They are without the organs of nutrition, &c., which characterize their parents, but the ovum nearly always possesses, stored up within its protoplasm, a greater or less quantity of vitelline matter or food-yolk, while the spermatozoon possesses in almost all cases the power of locomotion.