Archive image from page 61 of The development of the chick. The development of the chick : an introduction to embryology . developmentofchi02lill Year: 1936 44 THE DEVELOPMENT OF THE CHICK everywhere in the hen's egg. In a section of a germinal disc, showing the accessory cleavage (Fig. 20), it is seen that the peripheral boundary of the marginal cells cuts under the margin for a considerable distance. The accessory cleavage becomes manifest at the time of appearance of the first cleavage plane, and increases in amount Fig. 17. — Photograph of an eight-celled pigeon ovum (after Mary Blount
Archive image from page 61 of The development of the chick. The development of the chick : an introduction to embryology . developmentofchi02lill Year: 1936 44 THE DEVELOPMENT OF THE CHICK everywhere in the hen's egg. In a section of a germinal disc, showing the accessory cleavage (Fig. 20), it is seen that the peripheral boundary of the marginal cells cuts under the margin for a considerable distance. The accessory cleavage becomes manifest at the time of appearance of the first cleavage plane, and increases in amount Fig. 17. — Photograph of an eight-celled pigeon ovum (after Mary Blount). Accessory cleavage (ac. cl.) in the marginal zone bounding the segmented area. Vesicles, appearing black in the photograph, are seen on the surface of the yolk beyond the mar- ginal zone of the germinal disc. Orientation as in Fig. 18. up to about the32-celled stage, and thereafter gradually decreases until it completely disappears (Figs. 18 B, C, and D). The peripheral boundaries of the marginal cells disappear pari passu, and, when the accessory cleavage is finally wiped out, the mar- ginal cells are everywhere continuous with the periblast, as in the hen's egg (Figs. 18 B and C). In some eggs the accesso-y cleavage is much more extensive than in others; indeed, in some it appears to be entirely absent, but this is relatively rare. In the stage shown in Fig. 18 B, for instance, there is usually con- siderable accessory cleavage; but in this egg there is none. The variation is obviously due to variations in the number of super- numerary spermatozoa, such as may readily occur.
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