DEVELOPMENT OF THE CHICK 121 



embryo has a definite and nearly constant 

 orientation in relation to the long axis of the 

 egg. However the egg may lie, the developing 

 embryo, like the germinal disc which it replaces, 

 will be above. The long axis of the embryo, 

 moreover, lies at right angles to the long axis 

 of the egg, and if the latter be placed on a 

 table with its broad end away from the observer 

 the head end of the embryo will be towards the 

 right. Though this position is not absolutely 

 constant, it is sufficiently so to render it possible 

 with considerable certainty to distinguish the 

 anterior and posterior ends of the blastoderm 

 before a definite embryo has made its appear- 

 ance. 



Mesoderm. — During the early hours of in- 

 cubation a third layer of the blastoderm begins 

 to form between the ectoderm and the 

 entoderm. This is the mesodenn (Fig. 57), and 

 consists of cells derived from the thickened 

 ectoderm which forms the primitive streak 

 and from the so-called head -process of the 

 blastoderm immediately in front of the streak. 

 The mesoderm gradually extends to the edge of 

 the pellucid area and then invades the opaque 

 area (Fig. 56). 



In the mesoderm of the opaque area blood- 



