86 THE DEVELOPMENT OF THE CHICK 



with instruments of greater precision one cannot feel certain of 

 the results of such experiments. 



III. The Mesoderm of the Opaque Area 

 We have seen that the mesoderm arises from the sides of the 

 head-process and the primitive streak, and grows out between 

 the ectoderm and the entoderm to the margin of the pellucid 

 area; it then begins to overlap the opaque area at first behind, 

 later at the sides, appearing between the ectoderm and the germ- 

 wall. Figs. 44 A, B, C, and 45 illustrate its peripheral extension; 

 at first it spreads most rapidly behind the embryo, but soon ex- 

 tends with equal speed opposite the primitive streak, and thus 

 a considerable portion of the area opaca becomes three-layered, 

 consisting of ectoderm, mesoderm, and germ-wall (Figs. 40 C 

 and 41). The contour of the anterior margin of the mesoderm 

 iSa'^ first rounded, convex anteriorly (Figs. 44 A and B). Then 

 the antero-lateral angles of the mesoblast begin to extend forward 

 so that the anterior boundary becomes concave (Fig. 44 C) ; the 

 lateral horns thus established continue to grow forward and 

 ultimately meet in front of the head (Fig. 45) ; they thus bound a 

 mesoblast-free area in front of and beneath the head, known 

 as the proamnion, into which the mesoderm does not penetrate 

 until a relatively late stage of development. 



Blood-islands (Figs. 44 C and 45) develop early in the three- 

 layered part of the opaque area; appearing first behind the em- 

 bryo, they rapidly differentiate forward opposite the sides of 

 the embryo and follow the expansion of the mesoblast. This 

 three-layered portion of the opaque area is known as the vascular 

 area (area vasculosa) after the appearance of the blood-islands. 

 It soon acquires a very definite peripheral boundary by the forma- 

 tion of the vena (sinus) terminalis at its margin (Fig. 45). The 

 two-layered peripheral portion of the opaque area is known as 

 the vitelline area (area vitellina), and here again we distinguish 

 two zones, an outer including the zone of junction, and an inner 

 one (Figs. 32, 33). 



The first blood-islands are masses of cells lying on the germ- 

 wall behind the embryo; the first blood-cells (erythrocytes) and 

 blood-vessels arise from them, hence their name. Soon after 

 their origin the blood-islands appear red owing to the formation 

 of haemoglobin. Between the blood-islands and the ectoderm 



