Fig.14 The human ovum. Fig. 14—The human ovum, taken from the female ovary, magnified. The whole ovum is a simple round cell. The chief part of the globular mass is formed by the nuclear yelk (_deutoplasm_), which is evenly distributed in the active protoplasm, and consists of numbers of fine yelk-granules. In the upper part of the yelk is the transparent round germinal vesicle, which corresponds to the _ nucleus._ This encloses a darker granule, the germinal spot, which shows a _nucleolus._ The globular yelk is surrounded by the thick transparent germinal membrane (_ovolemma,_ or _ zona pellucida_). This is traversed by numbers of lines as fine as hairs, which are directed radially towards the centre of the ovum. These are called the pore-canals; it is through these that the moving spermatozoa penetrate into the yelk at impregnation.
When the mature bird-ovum has left the ovary and been fertilised in the oviduct, it covers itself with various membranes which are secreted from the wall of the oviduct. First, the large clear albuminous layer is deposited around the yellow yelk; afterwards, the hard external shell, with a fine inner skin. All these gradually forming envelopes and processes are of no importance in the formation of the embryo; they serve merely for the protection of the original simple ovum. We sometimes find extraordinarily large eggs with strong envelopes in the case of other animals, such as fishes of the shark type. Here, also, the ovum is originally of the same character as it is in the mammal; it is a perfectly simple and naked cell. But, as in the case of the bird, a considerable quantity of nutritive yelk is accumulated inside the original yelk as food for the developing embryo; and various coverings are formed round the egg. The ovum of many other animals has the same internal and external features. They have, however, only a physiological, not a morphological, importance; they have no direct influence on the formation of the fœtus. They are partly consumed as food by the embryo, and partly serve as protective envelopes. Hence we may leave them out of consideration altogether here, and restrict ourselves to material points—_to the substantial identity of the original ovum in man and the rest of the animals_ (Fig. 13).