DEVELOPMENT OF THE EMBRYO 453 



Figure 260 presents a diagrammatic scheme which makes it possible 

 to see the general outlines of gastrulation in eggs with varying quantities 

 of yolk. 



The zone of junction, where the peripheral region of the blastoderm 

 remains attached to the yolk, is called the area opaca, because when the 

 blastoderm is removed from the yolk-surface for laboratory study, the 

 yolk is so closely attached to this region that it adheres to the blastoderm 

 and renders the area more opaque. The more central portion which 

 thus has no yolk attached, is more translucent and is therefore called the 

 area pellucida. 



The area opaca later differentiates into the following three more or 

 less distinct zones (Fig. 261) : 



(1) The margin of overgrowth: Being a peripheral zone where 

 rapid proliferation pushes the cells out over the yolk without their ad- 

 hering to it. 



(2) The zone of junction: Which has an intermediate zone in 

 which the deeper lying cells have no complete cell boundaries, so that 

 they form a syncytium which blends (without a definite boundary) with 

 the superficial layer of white yolk to which it adheres by many pene- 

 trating strands of cytoplasm. 



(3) The germ wall: Being an inner zone made up of cells derived 

 from the inner border of the zone of junction, which have acquired defi- 

 nite boundaries and become more or less free from the yolk. Numerous 

 small yolk granules are usually found in the germ wall, due to the fact 

 that these were contained in the cytoplasm when they were still con- 

 nected with the yolk as cells of the zone of junction. It is the inner 

 margin of the germ wall which separates the area opaca from the area 

 pellucida. 



When the chick embryo is ready for gastrulation, there is a thinning 

 of the blastoderm at the caudal margin with a consequent freeing of the 

 blastoderm at the caudal margin from the yolk (Fig. 259, D). In a sur- 

 face view, the crescent shaped gap in the posterior quadrant of the zone 

 of junction marks the separation of the blastoderm from the yolk (Fig. 

 259, A). The blastopore is that region where the blastoderm is free 

 from yolk and where it is likewise very thin. 



It will be remembered that cell proliferation is continuous through- 

 out the entire blastoderm. The surface extent has now become much 

 greater by a general spreading out of the peripheral margins over the 

 yolk, but this extension, while taking place uniformly at the margins, 

 varies at the blastopore. This being at the posterior free end of the 

 blastoderm, the cells, as they proliferate, grow inward to form the ento- 

 derm. Once this differentiation has taken place, the part of the margin 



