OUTLINE OF DEVELOPMENT, CHRONOLOGY 63 
into the body-cavity through the umbilicus, which thereupon 
closes. The young chick usually hatches on the twenty-first day. 
Orientation. It is an interesting and important fact that 
the embryo appears in a definite relation to the line drawn through 
the axis of the entire egg, or to the line joining the bases of the 
two chalaze, which is usually the same thing. If the egg be 
placed as in Fig. 32 A, with the blunt end to the left, the head 
of the embryo will be found directed away from the observer 
when the blastoderm is above; the left side of the embryo is 
therefore towards the broad end, and the right side towards the 
narrow end of the egg. According to Duval this orientation is 
Fie. 33. — A. Yolk of hen’s egg incubated 84 hours. (After Duval.) 
B. Embryo and membranes of the hen’s egg on the seventh day of incu- 
bation. (After Duval.) 
Al., Allantois. Am., Amnion. a. v., (in B) Area vitellina. E., Embryo. 
S.t., Sinus terminalis. Other Abbreviations as in Fig. 32. 
found in about 98.5 % of eggs: of 166 eggs observed, in which 
the embryo was formed, Duval found 124 oriented exactly in 
this manner, 39 in which the axis of the embryo was slightly 
oblique, 2 in which the head was towards the broad end, and 1 
in which the usual position was completely inverted. In the 
pigeon’s egg the orientation of the embryo is equally definite, but 
shghtly different. The axis of the embryo cuts the axis of the 
entire egg at an angle of about 45°, the head of the embryo being 
