l68 PRINCIPLES OF EMBRYOLOGY 



downwards when it arrives there, and moves away again in a lower layer 

 which lies beneath the surface but above the endoderm, and which is there- 

 fore the mesoderm (Fig. 9.11). The first mesoderm to pass through the 

 streak migrates, not only laterally, but also towards the anterior. It does not 

 form part of the mesoderm of the embryonic axis, but finally lies well 

 out to the side. This is in strong contrast to what happens in Amphibia, 

 where the first invaginated mesoderm remains in the dorsal axis. But the 

 difference is merely one of the place at which the invagination of meso- 

 derm begins; in the Amphibia this is the dorsal lip, wliile in the birds the 

 first part of the primitive streak is in a region corresponding more nearly 

 to the ventral or lateral lips. 



— Epiblast 



. — Mesoderm 

 — Endoderm 



Figure 9.11 



Semi-diagrammatic section through the primitive streak of the chick, to 

 show the invagination of mesoderm. 



Very soon after, or perhaps even simultaneously with, the beginning 

 of this mesoderm-invagination, another movement affects the region of 

 the primitive streak. This is a streaming forwards along the streak to- 

 wards its anterior tip. As this movement gets under way, the streak length- 

 ens, and pushes out across the area pellucida. Whereas when it first appears 

 it occupies only a fifth, or less, of the diameter, it eventually comes to 

 extend more than halfway across the still circular area. While the forward 

 movement is still continuing in the anterior region, the posterior part of 

 the streak starts to push out backwards. When this double movement is at 

 its height, the streak is elongating so fast that it draws out the area pellu- 

 cida from its original circular shape into an oval or pear-shaped form 

 (Fig. 9.12). The movement bends the regions of the prospective map 

 into long arcs lying on each side of the streak, and since the invagination is 

 continuing all the time, by the close of the forward streaming nearly all 

 the lateral mesoderm has disappeared in the anterior region, leaving the 

 streak bordered by prospective somite material, with a small arc of pro- 

 spective chorda at the most anterior end (Fig. 9.7, p. 160). The anterior end 

 of the streak becomes somewhat more markedly thickened than the remain- 



