The fertilised bird-ovum (Fig. 15) is notably different. It is true that in its earliest stage (Fig. 13 E) this ovum also is very like that of the mammal (Fig. 13 F). But afterwards, while still within the oviduct, it takes up a quantity of nourishment and works this into the familiar large yellow yelk. When we examine a very young ovum in the hen’s oviduct, we find it to be a simple, small, naked, amœboid cell, just like the young ova of other animals (Fig. 13). But it then grows to the size we are familiar with in the round yelk of the egg. The nucleus of the ovum, or the germinal vesicle, is thus pressed right to the surface of the globular ovum, and is embedded there in a small quantity of transparent matter, the so-called white yelk. This forms a round white spot, which is known as the “tread” (_cicatricula_) (Fig. 15 _b_). From the tread a thin column of the white yelk penetrates through the yellow yelk to the centre of the globular cell, where it swells into a small, central globule (wrongly called the yelk-cavity, or _latebra,_ Fig. 15 _d′_). The yellow yelk-matter which surrounds this white yelk has the appearance in the egg (when boiled hard) of concentric layers (_c_). The yellow yelk is also enclosed in a delicate structureless membrane (the _membrana vitellina, a_). Fig.13 Ova of various animals, executing amœboid movements. Fig. 13—Ova of various animals, executing amœboid movements, magnified. All the ova are naked cells of varying shape. In the dark fine-grained protoplasm (yelk) is a large vesicular nucleus (the germinal vesicle), and in this is seen a nuclear body (the germinal spot), in which again we often see a germinal point. Figs. _A1–A4_ represent the ovum of a sponge (_Leuculmis echinus_) in four successive movements. _B1–B8_ are the ovum of a parasitic crab (_Chondracanthus cornutus_), in eight successive movements. (From _Edward von Beneden._) _C1–C5_ show the ovum of the cat in various stages of movement (from _ Pflüger_); Fig. _D_ the ovum of a trout; _E_ the ovum of a chicken; _F_ a human ovum. As the large yellow ovum of the bird attains a diameter of several inches in the bigger birds, and encloses round yelk-particles, there was formerly a reluctance to consider it as a simple cell. This was a mistake. Every animal that has only one cell-nucleus, every amœba, every gregarina, every infusorium, is unicellular, and remains unicellular whatever variety of matter it feeds on. So the ovum remains a simple cell, however much yellow yelk it afterwards accumulates within its protoplasm. It is, of course, different, with the bird’s egg when it has been fertilised. The ovum then consists of as many cells as there are nuclei in the tread. Hence, in the fertilised egg which we eat daily, the yellow yelk is already a multicellular body. Its tread is composed of several cells, and is now commonly called the _germinal disc._ We shall return to this _discogastrula_ in Chap. IX.