structure and function have arisen—a subject to which we shall return later. But the developmental processes which lead to the formation of fully developed ova and spermatozoa come to a full stop before fertilization and they do not usually begin again until a spermatozoon has entered an ovum, or until the latter has been stimulated by some other outside means. In some animals and plants eggs may develop regularly without fertilization, the stimulus to development being supplied by
Fig. 23. Diagrams of the Maturation and Fertilization of the Egg of a Mollusk (Crepidula). A, B. First maturation division (1st Mat. Sp.) C. Second maturation division (2d Mat. Sp.) and first polar body (1st PB) resulting from first division. ♂N, sperm nucleus, ♂C, sperm centrosome. D. Approach of sperm nucleus (♂N) and sperm (♂S) to egg nucleus (♀N) and sphere (♀S); the second polar body (2dPB) has been formed and the forest has divided (1stPB). E. Meeting of egg and sperm nuclei and origin of cleavage centrosomes. F. First cleavage of egg showing direction of currents in the cell.
