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. 
