=Spermatogenesis.=—The spermatozoa are formed by the seminiferous tubules of the testis. From these cells, called the _spermatogonia_, are formed other cells called _spermatocytes_, which in turn form the _spermatids_, the immediate forerunners of the spermatozoa. During the period of multiplication the spermatogonia divide repeatedly by mitosis. Numbers of small cells are thus produced, each containing in its nucleus the number of chromosomes typical of the somatic cell of that fowl. In the second period the cells become larger and spermatocytes of the first order are formed. Then comes the period of maturation, during which two succeeding divisions rapidly occur. The first division results in the formation of two cells exactly alike. These are spermatocytes of the second order. They differ from the somatic cells in that they contain only one-half the typical number of chromosomes. The second division produces two similar spermatids from one spermatocyte of the second order. Therefore four spermatids exactly alike may be formed from one spermatocyte of the first order. From these spermatids the spermatozoa are formed (Fig. 55). The heads of the spermatozoa contain the nuclei derived from the spermatids; the necks contain the centrosomes; and the tail, consisting of three parts is probably formed from the protoplasm. Three parts of the tail are as follows: first, the pars conjunctionis, which unites the tail to the neck; second, the pars principalis, which constitutes the main length of the tail; and, third, the pars terminalis, which consists of an axial filament which transverses the entire tail and is surrounded by a protoplasmic sheath. =Oögenesis.=—The ovum during its formation passes through three stages. [Illustration: FIG. 81.—Section of ovum in a hen. 1. Nucleolus. 2. Nucleus. 3. Liquor folliculi. 4. Stratum granulosum. 5. Follicular cells. 6. Theca folliculi. 7. Peripheral stroma. ] The first stage, that of _division_, takes place before the chick is hatched, and, according to Bradley, comes to an end about the time of hatching. This stage consists of the rapid formation of ova in the female chick. In the second stage, which begins about the time of hatching, there is an increase in the size of the units of the ovary, accompanied by _yolk formation_. At this time each ovum is in its own follicle (Fig. 81), and is surrounded by a layer of cuboidal cells and a _theca_. The theca is formed from the adjacent fibrous stroma. The third stage, that of _maturation_, commences during the development of the yolk and is complete after it has escaped to the oviduct. Maturation consists of each cell’s dividing into two unequal parts. In each division the cell is split into a small cell known as the _polar body_, which is cast off and disappears, and a larger cell, which is the ovum proper. In this process half of the original chromosomes are cast off.