The Development of the AlHgator 255 



will show the medullary canal open both above 

 and below, as in Figure d,d, while sections farther 

 caudad pass through the short region where the 

 folds are fused, and we have the appearance repre- 

 sented in Figure 8e. In Figure 8/ is shown a section 

 passing posterior to the short, fused region of the 

 folds, and we again have the medullary canal open 

 both above and below. Figure 8g represents a 

 section through the tip of the bent-under region of 

 the medullary folds, which are here fused below 

 and open above. 



Figure 8/z passes through the posterior part of 

 the head-fold, between the limits of the fold of the 

 ectoderm and that of the entoderm. The medul- 

 lary groove {mg) is here very wide and compar- 

 atively shallow ; its walls are continued laterally as 

 the gradually thinning ectoderm {ec) . The enteron 

 {ent) is completely enclosed, and forms a large, some- 

 what compressed, thick -walled cavity. Between 

 the dorsal wall of the enteron and the lower side of 

 the medullary canal lies the notochord {nt) , a small, 

 cylindrical rod of closely packed cells derived, in this 

 region at least, from the entoderm. In the posterior 

 region of the embryo it is not possible to determine 

 with certainty the origin of the notochord, owing to 

 the close fusion of all three germ layers. Between 

 the wall of the enteron and the lower side of the ecto- 

 derm is a considerable mass of mesoderm (mes), 

 which here consists of more scattered and angular 

 cells than in the preceding section. 



