OUTLINE OF DEVELOPMENT, CHRONOLOGY 



63 



into the body-cavity through the umbiUcus, 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 chalazse, 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 



Fig. 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, AUantois. 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 

 slightly 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 



