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. Fig. 57 Diagram of discoid segmentation in the bird’s ovum. Fig. 57—Diagram of discoid segmentation in the bird’s ovum (magnified). 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. The invagination or the folding inwards of the bird-blastula takes place in this case also at the hinder pole of the subsequent chief axis, in the middle of the hind border of the round germinal disk (Fig. 59 _s_). At this spot we have the most brisk cleavage of the cells; hence the cells are more numerous and smaller here than in the fore-half of the germinal disk. The border-swelling or thick edge of the disk is less clear but whiter behind, and is more sharply separated from contiguous parts. In the middle of its hind border there is a white, crescent-shaped groove—Koller’s sickle-groove (Fig. 59 _s_); a small projecting process in the centre of it is called the sickle-knob (_sk_). This important cleft is the primitive mouth, which was described for a long time as the “primitive groove.” If we make a vertical section through this part, we see that a flat and broad cleft stretches under the germinal disk forwards from the primitive mouth; this is the primitive gut (Fig. 60 _ud_). Its roof or dorsal wall is formed by the folded upper part of the blastula, and its floor or ventral wall by the white yelk (_wd_), in which a number of yelk-nuclei (_dk_) are distributed. There is a brisk multiplication of these at the edge of the germinal disk, especially in the neighbourhood of the sickle-shaped primitive mouth. We learn from sections through later stages of this discoid bird-gastrula that the primitive gut-cavity, extending forward from the primitive mouth as a flat pouch, undermines the whole region of the round flat lens-shaped blastula (Fig. 61 _ ud_). At the same time, the segmentation-cavity gradually disappears altogether, the folded inner germinal layer (_ik_) placing itself from underneath on the overlying outer germinal layer (_ak_). The typical process of invagination, though greatly disguised, can thus be clearly seen in this case, as Goette and Rauber, and more recently Duval (Fig. 61), have shown.