S4 THE DEVELOPMENT OF THE CHICK 
ination. The shaded margin represents the zone of Junction, the 
unshaded portion of the margin represents the area of invagina- 
tion of the entoderm. The dotted contour represents the margin 
of the pellucid area. In A the middle of the area of invagination 
is marked 1, and corresponding points to the right and left 2, 3, 
and 4. In diagram B it is supposed that the margin of invagina- 
tion is turned forward at 1, and that the lateral portions are 
brought together as far as 2, thus producing a suture in the middle 
line 1-2 continuous with the margin 3-4. The zone of invagina- 
tion is correspondingly reduced in extent and the zone of junction 
increased. In diagram C the lateral lips of the zone of invagina- 
tion are represented as completely concresced, thus producing a 
median suture 1, 2, 3, 4, extending through the posterior half 
of the area pellucida to the margin. The zone of Junction is 
on the point of closing behind the line of concrescence which is 
the primordium of the primitive streak. In diagram D, finally, 
the opaque area has closed in behind the line of concrescence 
which occupies the hinder half of the pellucid area. 
To apply this theory to the actual data of the development, 
it is only necessary to assume that the entoderm separates from 
the ectoderm along the line of concrescence, and that the primi- 
tive streak arises subsequently along the same line. The actual 
demonstration of the truth of this conception cannot be furnished 
by observation alone, however detailed. It is, however, possible 
to test it by experiment, though difficult because the concrescence 
must take place, if at all, prior to laying. The strong support 
of the theory lies at present in the data of comparative embry- 
ology; in the lower vertebrates the mesoderm and entoderm 
are both formed from the margin of invagination. 
Summarizing the matter, we may say that in the chick gastru- 
lation is divided into two separate processes: the first is the in- 
vagination of the entoderm from the margin, and the second is 
the ingrowth (or invagination) of mesoblast and notochord from 
the primitive streak, which represents the coalesced lips of the 
margin of invagination; the primitive groove is therefore the 
expression of a second phase of invagination. 
The genetic relation of the primitive streak to the margin of 
the blastoderm is well illustrated by an abnormal blastoderm 
described by Whitman in which the primitive groove was con- 
tinued across the area opaca to a marginal notch at the posterior 
