5. _Sporogony._--This method of reproduction has been described by E. Metchnikoff in _Cunina_ and allied genera. In individuals either of the male or female sex, germ-cells which are quite undifferentiated and neutral in character, become amoeboid, and wander into the endoderm. They divide each into two sister-cells, one of which--the spore--becomes enveloped by the other. The spore-cell multiplies by division, while the enveloping cell is nutrient and protective. The spore cell gives rise to a "spore-larva," which is set free in the coelenteron and grows into a medusa. Whether sporogony occurs also in the polyp or not remains to be proved. 6. _Sexual Reproduction and Embryology._--The ovum of Hydromedusae is usually one of a large number of oögonia, and grows at the expense of its sister-cells. No regular follicle is formed, but the oöcyte absorbs nutriment from the remaining oögonia. In _Hydra_ the oöcyte is a large amoeboid cell, which sends out pseudopodia amongst the oögonia and absorbs nutriment from them. When the oöcyte is full grown, the residual oögonia die off and disintegrate. [Illustration: FIG. 47.--Budding from the Ectoderm (germinal epithelium) in _Margellium_. (After C. Chun.) A, The epithelium becomes two-layered. B, The lower layer forms a solid mass of cells, which (C) becomes a vesicle, the future endoderm, containing the coelenteric cavity (coel), while the outer layer furnishes the future ectoderm. D, E, F, a thickening of the ectoderm on the distal side of the bud forms an entocodon (Gc). G,H, Formation of the medusae. s.c, Sub-umbral cavity. r.c, Radial canal. st, Stomach, which in H acquires a secondary communication with the digestive cavity of the mother. cc, Circular canal. v, Velum. t, Tentacle.] The spermatogenesis and maturation and fertilization of the germ-cells present nothing out of the common and need not be described here. These processes have been studied in detail by A. Brauer [2] for _Hydra_.