13, a) are developed in the upper region of the megaspore, each consisting of a large egg-cell surmounted by two neck-cells and a canal-cell which is cut off shortly before fertilization.
A, The egg-cell has divided into two spheres, of which the lower contains more food-material, whilst the upper is again incompletely divided into two smaller spheres.
During the development of the egg-cell, food material is transferred from these cells through the pitted wall of the ovum.
Self-fertilization occurs when the pollen of a given flower affects the egg-cell of the same individual flower.
Polyembryony is generally associated with the development of cells other than the egg-cell.
- Early Stages of division of the Fertilized Egg-cell in Nassa mutabilis.
But it was only after Darwin that the cell-theory of Schwann was extended to the embryology of the animal kingdom generally, and that the knowledge of the development of an animal became a knowledge of the way in which the millions of cells of which its body is composed take their origin by fission from a smaller number of cells, and these at last from the single egg-cell.
Of the three cells at the micropylar end of the sac, all naked cells (the so-called egg-apparatus), one is the egg-cell or oosphere, the other two, which may be regarded as representing abortive egg-cells (in rare cases capable Of fertilization), are known as synergidae.
In Taxus the body-cell eventually divides into two, in which the products of division are of unequal size, the larger constituting the male generative cell, which fuses with the nucleus of the egg-cell.
For an account of the further development of the megaspore, and the formation of the egg-cell, from which after fertilization is formed the embryo, see Gymnosperms and Angiosperms.