. The development of the chick; an introduction to embryology. Birds -- Embryology. DEVELOPMENT PRIOR TO LAYING 57 anterior position in the entoderm (Patterson). But after the margin has thickened the farther extension of the entoderm is due, largely at least, to ingrowth from the marginal thickening. Patterson also believes that the thickening of the margin is due not so much to multiplication of cells in situ as to immigration of cells from the sides. This view is also supported by experi- ments. ^ E G. Fig. 29. — Diagrammatic reconstruction of the blastoderm of a pigeon's egg, 36 hours afte
. The development of the chick; an introduction to embryology. Birds -- Embryology. DEVELOPMENT PRIOR TO LAYING 57 anterior position in the entoderm (Patterson). But after the margin has thickened the farther extension of the entoderm is due, largely at least, to ingrowth from the marginal thickening. Patterson also believes that the thickening of the margin is due not so much to multiplication of cells in situ as to immigration of cells from the sides. This view is also supported by experi- ments. ^ E G. Fig. 29. — Diagrammatic reconstruction of the blastoderm of a pigeon's egg, 36 hours after fertilization; from the same series as Fig. 28. X (After Patterson.) E., Invaginated or gut entoderm. O., Margin of overgrowth. PA., Outer margin of pellucid area. R., Margin of invagmation (dorsal lip of blastopore). S., Beginning of yolk-sac entoderm. Y., Yolk zone. Z., Zone of junction. , ,. . The arrows at the posterior margin indicate the direction of movement of the halves of the margin. The circles in the pellucid area indicate yolk masses in the segmentation cavity. Figure 29 is a reconstruction of a blastoderm in the stage of Fig. 28, that is at the height of gastrulation. The margin of overgrowth (cf. Fig. 28 O) is represented by the area O; the zone of junction by the ruled area Z; the inner portion of the. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the original Lillie, Frank Rattray, 1870-1947. New York, Holt
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