Archive image from page 59 of The development of the chick. The development of the chick : an introduction to embryology . developmentofchi02lill Year: 1936 42 THE DEVELOPMENT OF THE CHICK well as deep boundaries; they may be called the marginal cells (Fig. 16 D). The distinction between central and marginal cells is one of great importance which should be clearly grasped. In the surface views of later cleavages the following points should be noted: (1) the group of central cells increases by the addition of cells cut off from the inner ends of the marginal cells, and by the multiplication of
Archive image from page 59 of The development of the chick. The development of the chick : an introduction to embryology . developmentofchi02lill Year: 1936 42 THE DEVELOPMENT OF THE CHICK well as deep boundaries; they may be called the marginal cells (Fig. 16 D). The distinction between central and marginal cells is one of great importance which should be clearly grasped. In the surface views of later cleavages the following points should be noted: (1) the group of central cells increases by the addition of cells cut off from the inner ends of the marginal cells, and by the multiplication of the central cells themselves; (2) the marginal cells increase by the formation of new radial furrows. The increase of the central cells is much more rapid than that of Fig. 16 A. Median section of a blastoderm of the hen's egg which showed about 64 cells in surface view (after Patterson). , segmentation cavity. the marginal cells, and the cells themselves are much smaller than the marginal cells, both because of their mode of origin and also because of their more rapid multiplication. The area of the central cells is also constantly increasing, with consequent re- duction of the marginal zone (Fig. 16 E). Emphasis has been laid by several authors on the excentric position of the smallest cells, and the inference has been drawn that these represent the hinder end of the glastodisc. Similar excentricity in the pigeon's egg is without reference to the future embryonic axis (see Fig. 18). But the surface views do not show what is going on in the deeper parts of the germinal disc. At the eight-celled stage a narrow space appears in the depth of the central portion of the blastoderm approximately between protoplasm and yolk; this is
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