108 OUTLINES OF CHORDATE DEVELOPMENT 



pleted ventrally, its dorsal region, in the more rapidly develop- 

 ing dorsal lip, has broadened considerably so that its general 

 form might be compared with a signet ring. 



As the blastopore is completed and begins to close, the con- 

 fluence of its lateral margins transports masses of the cells 

 toward the mid-line, and leaves them as a broad median band 

 extending upward from the dorsal lip of the blastopore. As a 

 result of the multiplication of these cells, and of the downward 

 extension of this lip, a considerable axial thickening is formed. 

 And at the same time the extension of the archenteron carries 

 the yolk cells, with which the mesoderm is closely related on one 

 side, upward toward the animal pole, so that altogether, even 

 in these comparatively early stages, the extent of the mesoderm 

 is nearly as great as that of the endoderm. And soon the meso- 

 derm is definitely delimited from the endoderm by a rearrange- 

 ment of cells giving the appearance of an irregular delamination 

 (Fig. 33, Z>). This delamination commences in the dorso- 

 lateral regions either side of the thickened axial mass and gradu- 

 ally extends thence anteriorly and laterally around the sides of 

 the archenteron separating a thin layer of definitive endoderm 

 walling the gut cavity, and a much thicker mass of mesoderm 

 between this and the superficial ectoderm (Fig. 44). In the 

 region of the lower pole of the gastrula, under the thickest mass 

 of yolk, the delamination comes to the surface of the yolk mass 

 forming a free circular margin of mesoderm. From this free 

 margin cells and groups of cells break or bud off passing farther 

 ventrally, and finally reaching the lower pole, completing thus 

 a fairly continuous layer between ectoderm and endoderm 

 (yolk) (Figs. 44, 61, 63). In the region of the dorsal axial 

 mass, particularly in the region of the blastopore, the de- 

 lamination is delayed and its course somewhat modified. In 

 these regions the cells concerned in mesoderm formation have 

 very different relations from the remainder, since throughout 

 they are the derivatives of cells which have been invaginated 

 from the outer layer. And here too the development of the 

 notochord complicates the matter somewhat. 



Sections cut transversely through the blastopore and the 



