. The development of the chick; an introduction to embryology . o theintestine of the embryo. This stalk runs through an openingin the ventral body-wall, the umbilicus, where the amnion, whichhas developed from the extra-embryonic somatojiletire. joins thebody-wall (Fig. 33 H). About the ninetecMith dav of incubation the Aolk-sac is drawn OUTLINE OF DEVELOPMENT, CHRONOLOGY 63 into the body-cavity through the iinihiliciis, which thereuponcloses. The young chick usually hatches on the twenty-first It is an interesting and important fact thatthe embryo appears in a definite relat
. The development of the chick; an introduction to embryology . o theintestine of the embryo. This stalk runs through an openingin the ventral body-wall, the umbilicus, where the amnion, whichhas developed from the extra-embryonic somatojiletire. joins thebody-wall (Fig. 33 H). About the ninetecMith dav of incubation the Aolk-sac is drawn OUTLINE OF DEVELOPMENT, CHRONOLOGY 63 into the body-cavity through the iinihiliciis, which thereuponcloses. The young chick usually hatches on the twenty-first It is an interesting and important fact thatthe embryo appears in a definite relation to the line drawn throughthe axis of the entire egg, or to the line joining the bases of thetwo chalazae, which is usually the same thing. If the egg beplaced as in Fig. 32 A, with the blunt end to the left, the headof the embryo will be found directed away from the observerwhen the blastoderm is above; the left side of the embryo istherefore towards the broad end, and the right side towards thenarrow end of the egg. According to Duval this orientation is. Fig. .VA. —A. Yolk of hens egiS incubated 84 hours. (After Duval.)B. Embryo and membranes of the hens egg on the seventh day of incu-bation. (After Duval.) Al., Allantois. Am., Amnion, a. v., (in B) Area vitellina. E., t., Sinus terminalis. Other Ablireviations as in Fig. 32. found in about of eggs: of 166 eggs observed, in whichthe embryo was formed, Duval found 124 oriented exactly inthis manner, 39 in which the axis of the embryo was slightlvobliciue, 2 in which the head was towards the broad end, and 1in whicli the usual position was completely inverted. In thepigeons egg the orientation of the embryo is eqiuilly definite, butslightly different. The axis of the embryo cuts the axis of theentire egg at an angle of about 45°, the head of the embrvo being 64 THE DEVELOPMENT OF THE CHICK directed away from the observer to the right;, when the broadend of the egg is to the observers left as in Fig. 32 A.
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