Fig 5.—Diagram of the life cycle of a backboned animal. such as a fish or a mammal. illustrating the union at eng and sperm. F. and S. to form the zygote, Z, the origin of the phorozoon or asexual generation (trophoblast), the germinal track trim Z to U K Z, the latter being the primitive germ-cell. The divisions of the latter are carried to seven mitoses, or cell-divisions, as in some male dog-fish (in a potential female embryo there would be an additional division, giving 256 primary germ-cells). Diagramatically, the unfolding of one primary germ-cell, the 66th, is depicted as, forming an embryo or sexual generation. To complete the track of heredity from generation to generation through the morphological continuity of the germ-cells a diagram of oogenesis or egg formation has been appended to the 96th germ-cell. and one of spermatogenesis or sperm-formation to the 32nd. In the former the formation of a male egg and of a female egg is shown in the latter the two forms of sperms (as in the fresh-water snail, Pa u lina. after the statements of F. Meves), i.e. the hairlike or functional and, the wormlike or functionless sperm. The additional division in the formation of functional male egg should be noted. It accords with the additional division to form primary germ-cells in the development of a female. page 124fig5