30 EARLY EMBRYOLOGY OF THE CHICK 



brought into apposition in the mid line. Figures C, and D, 

 show how, by the continuation of the same converging growth, 

 the edges of the blastopore are folded together into a line of 

 fusion at right angles to the Original marginal position of the 

 blastopore. At the completion of concrescence, the germ wall of 

 the blastoderm has coalesced posterior to the blastopore leaving 

 the line along which the blastopore lips have fused within the 

 area pellucida. The non-committal term primitive streak was 

 given to this structure before its origin by fusion of the lips of 

 the blastopore was suspected. 



The Formation of the Mesoderm. In its early condition the 

 primitive streak is a scarcely recognizable thickening of the 

 blastoderm marking the line of fusion of the lips of the blasto- 

 pore. The well defined groove with thickened ridges on either 

 side, seen in chicks of 15 to 1 6 hours incubation, is a later devel- 

 opment. A new process, the formation of the mesoderm, is 

 taking place at this region and the change in the configuration 

 of the primitive streak is its outward manifestation. It will 

 be recalled that the lip of the blastopore is in all forms a region 

 of rapid cell proliferation. It is a region from which we can 

 trace the addition of cells to the differentiated germ layers, but 

 it is itself indifferent. Ectoderm and entoderm both merge 

 into this indifferent area at the lip of the blastopore. It is 

 impossible to fix, except arbitrarily, where ectoderm begins and 

 entoderm ends. Later when the mesoderm appears, we can 

 trace the origin of its cells directly or indirectly to the same 

 area of indifferent, rapidly proliferating cells. It is therefore 

 wholly in line with the embryology of other forms to find the 

 mesoderm of the chick arising at the fused lips of the blastopore. 



The manner in which the mesoderm arises can be understood 

 only by the study of sections or diagrams of sections. Figure 

 10, A, represents schematically the conditions which would be 

 seen in a section cut in the line b-b across the marginal notch 

 of an embryo of the stage depicted in Figure 9, B. The mar- 

 gins of the blastopore at the point where this section is located 

 have been folded so they lie in close proximity to each other. 

 A little later they would be fused as shown in Figure 10, B. 

 At the region of fusion, that is to say at the primitive streak, 

 the entoderm and ectoderm merge in a mass of rapidly dividing 

 cells (Fig. 13, D). A section across the primitive streak at a 



