If a father is prepotent, he may have a greater effect in producing the formed child than the mother has, and _vice versa,_ as when a son closely resembles his father or his mother. {72} Prepotency, moreover, may extend down through generations and centuries. In the streets of Palermo to-day typical Normans may be seen, despite the intermarriages of centuries, who are the descendants of those male Normans that went down to Sicily with Tancred. There are Romans there, too, and Saracens. When the Belgae--a race of tall, red-bearded men, with elliptical skulls--went from the continent of Europe to Ireland, probably six centuries before our era, they conquered the aborigines, a gentle, brune race of lower stature. These Belgae became the ancestors of the chieftain class, and their physical type persists until to-day; so does that of the Pictish aborigines. Daniel O'Connell had a typical Belgic body. Other big, blond Irishmen are Norse or Danish in remote origin. How is the extremely complex human body with its various physical characteristics built up from the nucleus of a fecundated cell, the ovum? The endeavour to answer this question has brought out most ingenious speculation from nearly all the great biologists of modern times. The question is the foundation of the theories of heredity, and it is also fundamental in the theories of evolution. 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. Within the vitelline membrane is a granular cytoplasm, the vitellus (yolk), and in this lies the nucleus, which in the old text-books was called the germinal vesicle. This nucleus contains a nucleolus. The human spermatozoon consists of a flattened head which has a thin protoplasmic cap extending down two-thirds of its length. In the head is the nucleus with the chromatin. Beyond the head is the neck, which contains the anterior and posterior centrosomes. Behind the neck is the tail, or flagellum, in three parts,--the middle piece, the principal part, and the end piece. From the neck to the end of the tail centrally runs a bundle of fibrils, the axial filament. In the middle piece these fibrils are wrapped within a single spiral filament which winds from the neck down to the annulus at the beginning of the principal part, and lies in a clear fluid. Without the spiral filament, along the middle piece, is the mitochondria, a finely granular protoplasmic layer. The principal part of the tail consists of the axial {73} filament enclosed in an involucrum, and the end piece is made up of this filament without the involucrum.