8. First female form: the cap-like anterior part has become detached and the egg-cells (_eiz_) are escaping. 9. Second female form: _eiz_ = egg-cells; outside these are the muscular layer (_m_) and the ectoderm (_ekt_). 10 and 11. Two fragments of such a female broken to pieces by spontaneous division: the egg-cells are embedded in a granular mass, and undergo embryonic development in it at a later period; the whole is surrounded by ciliated cells. 12. Male discharging the spermatozoa by the breaking up of the ectoderm (_ekt_); _sp_ spermatozoa; _m_ muscle.] There is nevertheless no doubt that in the first female form, when sexually mature, the greater part, not only of the endoderm but of the whole body, is made up of ova, so that the animal resembles a thin-walled sac full of eggs. The ova escape by the bursting of the thin ectoderm, and when they have all escaped, the thin disintegrated membrane, composed of ciliated cells, is no longer in a condition to live, and dies at once. This is the course of events as described by Götte, and he is probably correct in his interpretation. This is the real death of the Orthonectides, and if we regard them as low primitive forms (Mesozoa), here for the first time in the ascending series we meet with natural death. But the causes of this are scarcely so clear as Götte seems to think when he ascribes it to the effect of reproduction—an effect which is ‘not only empirically necessary, but absolutely unavoidable.’ Such a necessity is explained by the fact that the endoderm consists entirely of germ-cells. Now the life of the organism, being dependent upon the mutual action of both layers, must cease as soon as the whole endoderm is extruded during reproduction. Arguments such as these pass over the presence of a mesoderm; but apart from this omission, it does not appear to me so self-evident from a purely physiological standpoint, that the ectodermal sheath with its muscle layer must die after the extrusion of the germ-cells. In those females to which Götte refers in this passage, the whole sheath remains at first quite uninjured, with the exception of a small cap at the anterior end, which is pushed off to give exit to the ova; and inasmuch as the sheath continues to swim about in the nutritive fluids after this has taken place, the proof is at any rate wanting that it cannot support itself quite as well as before, although it has lost the germ-cells. Then why does it die? My answer to this is simple:—because it has lived its time; because its length of life is limited to a period which corresponds with the time necessary for complete reproduction. The physical constitution of the body is so regulated that it remains capable of living until the extrusion of the reproductive cells, and then dies, however favourable external conditions may be for its further support.