24 EARLY EMBRYOLOGY OF THE CHICK 



A comparative review of the diagrams of Figure 6 will afford 

 a general understanding of the infolding process of gastrulation. 

 These diagrams aim to convey merely the scheme of the process. 

 They are therefore simplified and emphasize the similarities 

 of gastrulation in forms with widely varying amounts of yolk, 

 rather than the details of the process in any one form. With 

 this general groundwork we may now profitably return to the 

 blastula stage and consider in somewhat more detail the process 

 of gastrulation as it occurs in birds. ' 



Gastnilation in Birds. — We have already established the 

 blastula as a disc of cells lying on the yolk but separated from it 

 centrally by a flattened blastocoele or segmentation cavity. 

 The peripheral part of the blastoderm where the marginal cells 

 lie unseparated from the yolk has been termed the zone of 

 junction (Fig. 7, Z>). This part of the blastoderm is also called 



► the area opaca because in preparations made by removing the 

 blastoderm from the yolk surface, yolk adheres to it and renders 



•it more opaque. This opacity is especially apparent, when a 

 preparation is viewed under the microscope by transmitted 

 light. The central area of the blastoderm, because it is sepa- 

 rated from the yolk by the segmentation cavity, does not bring 

 a mass of adherent yolk with it when the blastoderm is removed. 



.It is for this reason translucent and is called the area pellucida. 

 The area opaca later becomes differentiated so that three more 

 or less distinct zones may be distinguished: (i) a peripheral 

 zone known as the margin of overgrowth where rapid prolifera- 

 tion has pushed the cells out over the yolk without their becom- 

 ing adherent to it; (2) an intermediate zone known as the zone 

 of junction in which the deep-lying cells do not have complete 

 cell boundaries but constitute a syncytium blending without 

 definite boundary into the superficial layer of white yolk and 

 adhering to it by means of penetrating strands of cytoplasm; 

 (3) an inner zone known as the germ wall made up of cells 

 derived from the inner border of the zone of junction which have 

 acquired definite boundaries and become more or less free from 

 the yolk. The cells of the germ wall usually contain numerous 

 small yolk granules which were enmeshed in their cytoplasm 

 when they were, as cells of the zone of junction, unseparated 

 from the yolk (Fig. 7, B, E). The inner margin of the germ 

 wall marks the transition from area opaca to area pellucida. 



