84 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 



