The precise steps in the formation of the sperm have also been studied, and it has been determined that a parallel series of changes occur. The sperm, when it is fully formed, contains also one-half the number of chromosomes characteristic of the species. Now, egg and sperm are the two germinal elements which unite in development. Fertilization takes place by the union of sperm and egg, and inasmuch as the nuclei of each of these structures contain one-half of the number of chromosomes characteristic of the species, their union in fertilization results in the restoration of the original number of chromosomes. The fertilized ovum is the starting-point of a new organism, and from the method of its fertilization it appears that the parental qualities are passed along to the cells of every tissue. The complex mechanism exhibited in the nucleus during segmentation is very wonderful. The fertilized ovum begins to divide, the nucleus passing through a series of complicated changes whereby its chromosomes undergo a lengthwise division--a division that secures an equable partition of the substance of which they are composed. With each successive division, this complicated process is repeated, and the many cells, arising from continued segmentation of the original cell, contain nuclei in which are embedded descendants of the chromosomes in unbroken succession. Moreover, since these chromosomes are bi-parental, we can readily understand that every cell in the body carries both maternal and paternal qualities. The careful analysis of the various changes within the nuclei of the egg proves to be the key to some of the central questions of heredity. We see the force of the point which was made in a previous chapter, that inheritance is in the long run a cellular study, and we see in a new light the importance of the doctrine of germinal continuity. This conception, in fact, elucidates the general problem of inheritance in a way in which it has never been elucidated by any other means. For some time the attention of investigators was concentrated upon the nucleus and the chromosomes, but it is now necessary to admit that the basis of some structures is discoverable within the cytoplasm that surrounds the nucleus. Experimental observations (Conklin, Lillie, Wilson) have shown the existence of particular areas within the apparently simple substance of the egg, areas which are definitely related to the development of particular parts of the embryo. The removal of any one of these pre-localized areas prevents the development of the part with which it is genetically related. Researches of this kind, necessitating great ingenuity in method and great talents in the observers, are widening the field of observation upon the phenomena of heredity.