The sexual reproduction of the many-celled animals is always essentially alike.  A single one of the body cells is set apart to start the next generation, and this cell, after separating from the body of the animal or plant which produced it, begins to divide, as already shown in Fig. 8, and the many cells which arise from it eventually form the new individual This reproductive cell is the egg.  But before its division can begin there occurs in all cases of sexual reproduction a process called fertilization, the essential feature of which is the union of this cell with another commonly from a different individual.  While the phenomenon is subject to considerable difference in details, it is essentially as follows: 

[Illustration:  FIG. 33—­An egg showing the cell substance and the nucleus, the latter containing chromosomes in large number and a nucleolus]

The female reproductive cell is called the egg, and it is this cell which divides to form the next generation.  Such a cell is shown in Fig. 33.  Like other cells it has a cell wall, a cell substance with its linin and fluid portions, a nucleus surrounded by a membrane and containing a reticulum, a nucleolus and chromatic material, and lastly, a centrosome.  Now such an egg is a complete cell, but it is not able to begin the process of division which shall give rise to a new individual until it has united with another cell of quite a different sort and commonly derived from a different individual called the male.  Why the egg cell is unable to develop without such union with male cell does not concern us here, but its purpose will be evident as the description proceeds.  The egg cell as it comes from the ovary of the female individual is, however, not yet ready for union with the male cell, but must first go through a series of somewhat remarkable changes constituting what is called maturation of the egg.  This phenomenon has such an intimate relation to all problems connected with the cell, that it must be described somewhat in detail.  There are considerable differences in the details of the process as it occurs in various animals, but they all agree in the fundamental points.  The following is a general description of the process derived from the study of a large variety of animals and plants.

[Illustration Fig. 34.—­This and the following figures represent the process of fertilization of an egg.  In all figures cr is the chromosomes; cs represents the cell substance (omitted in the following figures); mc is the male reproductive cell lying in contact with the egg; mn is the male nucleus after entering the egg.]

[Illustration:  FIG. 35.—­The egg centrosome has divided, and the male cell with its centrosome has entered the egg.]