When the fertilisation of the bird's ovum has taken place within the mother's body, we find in the lens-shaped stem-cell the progress of flat, discoid segmentation (Figure 1.57). First two equal segmentation-cells (A) are formed from the ovum. These divide into four (B), then into eight, sixteen (C), thirty-two, sixty-four, and so on. The cleavage of the cells is always preceded by a division of their nuclei. The cleavage surfaces between the segmentation-cells appear at the free surface of the tread as clefts. The first two divisions are vertical to each other, in the form of a cross (B). Then there are two more divisions, which cut the former at an angle of forty-five degrees. The tread, which thus becomes the germinal disk, now has the appearance of an eight-rayed star. A circular cleavage next taking place round the middle, the eight triangular cells divide into sixteen, of which eight are in the middle and eight distributed around (C). Afterwards circular clefts and radial clefts, directed towards the centre, alternate more or less irregularly (D, E). In most of the amniotes the formation of concentric and radial clefts is irregular from the very first; and so also in the hen's egg. But the final outcome of the cleavage-process is once more the formation of a large number of small cells of a similar nature. As in the case of the fish-ovum, these segmentation-cells form a round, lens-shaped disk, which corresponds to the morula, and is embedded in a small depression of the white yelk. Between the lens-shaped disk of the morula-cells and the underlying white yelk a small cavity is now formed by the accumulation of fluid, as in the fishes. Thus we get the peculiar and not easily recognisable blastula of the bird (Figure 1.58). The small segmentation-cavity (fh) is very flat and much compressed. The upper or dorsal wall (dw) is formed of a single layer of clear, distinctly separated cells; this corresponds to the upper or animal hemisphere of the triton-blastula (Figure 1.45). The lower or ventral wall of the flat dividing space (vw) is made up of larger and darker segmentation-cells; it corresponds to the lower or vegetal hemisphere of the blastula of the water-salamander (Figure 1.45 dz). The nuclei of the yelk-cells, which are in this case especially numerous at the edge of the lens-shaped blastula, travel into the white yelk, increase by cleavage, and contribute even to the further growth of the germinal disk by furnishing it with food-stuff. (FIGURE 1.57. Diagram of discoid segmentation in the bird's ovum (magnified about ten times). Only the formative yelk (the tread) is shown in these six figures (A to F), because cleavage only takes place in this. The much larger food-yelk, which does not share in the cleavage, is left out and merely indicated by the dark ring without.)