In osseous fish the vitelline membrane is usually either absent or may perhaps in some instances, _e.g._ the Perch, be imperfectly represented. In the ripe ovum of the Herring there is a distinctly developed membrane external to the zona radiata which is probably the vitelline membrane. The zona radiata attains a very great development, and is generally provided with knobs of various shapes on its outer surface. A delicate membrane internal to this--my third membrane--has often been described, but there is still some doubt about its existence. In some cases an external less granular layer of the ovum itself has been described as a special membrane. In the Perch a peculiar mucous capsule, penetrated by irregular branched prolongations of the follicle cells, is present in addition to the ordinary membranes. In Petromyzon a zona radiata appears to be present, which in the adult is divided into two layers, both of them radiately striated according to Calberla, but according to Kupffer and Benecke the outer one is not perforated, and would appear therefore to be a vitelline membrane as defined above. A delicate membrane is formed at a comparatively late period around the ova of the Amphibia, and is stated (Waldeyer, No. 6, and Kolessnikow) to have a delicate radial striation. It probably corresponds with the zona radiata. In Mammalia a radiately striated membrane--the zona radiata--is generally described as being present, and internal to it, in the nearly ripe egg, a delicate membrane has been shewn by E. van Beneden to exist. Externally to the zona radiata there may be observed a granular membrane irregular on its outer surface on which the cells of the discus are supported. This membrane is more or less distinctly separated from the zona radiata; and by tracing back its development it appears very probable that it is the remnant of the first-formed membrane in the very young ovum, and therefore the vitelline membrane. A micropyle (first discovered by Ransom, No. 74) is present in a large number of osseous fish and in Petromyzon (Calberla). Doubts have been thrown on its existence in the latter form by Kupffer and Benecke; and at any rate it would only seem to perforate the zona radiata. In the osseous fish in which it has been detected, Salmonidæ, Percidæ (Gasterosteus), Clupeidæe, etc., it forms a minute perforation of the zona radiata at the animal pole, just large enough to admit a single spermatozoon. Its characters differ slightly in different cases, but there is usually a shallow depression, in the centre of which it is situated.