ao., aorta. B.C., Bowman's capsule. coe., coelom. d.g., ganglion on dorsal root of spinal nerve. gl., -its branch- [arteriole] to form glomerulus. g.r., genital ridge. I., intestine. M.D., Mullerian duct. ns. [nst.], nephrostome. n.c., notochord; -n.s.-, [n.sh.] its sheath. s.c., neural canal. W.D., Wolffian duct. {Illustration: Diagram Sheet 22.} Sheet 23. -The Development of the Fowl_. Figure 1. Diagram of the early ovum. The section below is a small portion of the blastodermic area. b.d., blastoderm. y., the undivided yolk. s.c., the segmentation between the blastoderm and yolk. Compare s.c. in {Sheet} 22, {Figure} 4. Figure 2. Area pellucida about the sixteenth hour. The figure below is the central part of the section indicated by the transverse line, and showing the primitive streak (p.s.). Figure 3. Area pellucida about the twenty-first hour. Two sections through a and b below. Figure 4. About the twenty-fifth hour; surface view; longitudinal section to right and transverse above. Figure 4b. Diagrammatic rendering of same stage (compare Figure 9 of Frog and 9.ii. Amphioxus). This will be most clearly understood if the reader look at Sheet 22, {Figure} 9, and imagine Y. enormously increased, and the embryo sinking into it. Epiblast, ep., -line of dashes- [black line]. Mesoblast, dotted. Hypoblast, -black- [line of dashes]. pp., the pleuro-peritoneal cavity. Figure 5 and 6 illustrate formation of amnion (a.) and allantois (all.). 6 is about the fourth day. {Illustration: Diagram Sheet 23.} Sheet 24. -The Development of the Fowl_. Figure 1. Chick about the -fifth- [third] day. At this stage the chick lies on its left side in the yolk. [For lettering of blood vessels, see (7) below.] i., the intestine. u.v., the yolk sac. v.v., the vitelline veins. al., the allantois. Figure 2. Chick about sixth day. Figure 3. Development of heart. Figure 4. Development of the eye. Figure 5. Chick about the sixteenth day. A.M. is the amnion surrounding the embryo. Note particularly how the allantois (al.) has spread over surface of shell and how the yolk sac is shrivelled. Figure 6. Figures to illustrate the relative function and importance of allantois and yolk sac in bird and mammal. In the fowl, however, the blood-vessels of the allantois also probably absorb the albumen of the egg, and may excrete urea into the egg-space. Figure 7. Simplified figure of the embryonic circulation, for comparison with the similar figures annexed to Dog-Fish and Rabbit. {Lines from Second Edition only.} [A.C., anterior cardinal. Ao., Aorta. Br4, sixth aortis arch (fourth branchial). C.S. Cuvierian sinus. H., the heart. I.C., inferior cava. P.C., posterior cardinal vein. Tr.A., truncus arteriosus. v.v., vitelline vein.] Figure 8. Chick on the nineteenth day. {Illustration: Diagram Sheet 24.}