128 THE DEVELOPMENT OF THE CHICK 



cavity peripherally. The external vitelline area is relatively 

 narrow, and consists (1) of the zone of junction adjoining the 

 internal vitelline area, and (2) a free margin separate from the 

 yolk (margin of overgrowth). The zone of junction is the per- 

 sistent embryonic or formative part of the blastoderm from 

 which the extra-embryonic ectoderm and entoderm arises. Thus 

 as it spreads peripherally over the surface of the yolk, it leaves 

 on its central margin the differentiated extra-embryonic ecto- 

 derm and entoderm; in other words, the zone of junction is the 

 youngest part of the blastoderm, and the concentric zones that 

 may be drawn within it represent successively older stages in a 

 centripetal direction. Therefore in a transverse section through 

 the entire blastoderm successive stages of differentiation of the 

 ectoderm and particularly of the entoderm are met as one passes 

 from the zone of junction towards the center. 



The free margin arises from the zone of junction in the manner 

 already described in Chapter II. It may be considered as a part 

 of the ectoderm and it terminates in a row of enlarged cells that 

 often exhibit amoeboid prominences on their margins. It would 

 appear that these cells have the function of a marginal wedge 

 that separates the vitelline membrane and yolk. 



The germ-wall has been the subject of many extended re- 

 searches, but a definitive solution of its origin and function has 

 not hitherto been obtained, mainly on account of the incomplete 

 knowledge of its early history. The ground here taken is in some 

 respects different from that of the various authors, but it is based 

 on a study of its early history given in Chapter II. There is no 

 deviation from the mode of formation of the zone of junction in 

 the stage under consideration from what was found in earlier 

 stages, and there is no reason to believe that its subsequent history 

 varies in any important respect. It appears to be produced by 

 continuous proliferation of the cells in the yolk as in earlier stages 

 (see Fig. 68 E). These cells actively engulf the large yolk gran- 

 ules, and the histological structure becomes in consequence diffi- 

 cult of analysis. The cells lose their individuality and constitute 

 an extended syncytium, the protoplasm of which is packed with 

 yolk-granules. In removing the blastoderm from the egg in salt- 

 solution one finds always, after removing the yolk that may be 

 washed off, a narrow submarginal zone of adherent yolk that is 

 not readily removed, and this is the site of the zone of junction. 



