60 THE DEVELOPMENT OF THE CHICK 



to lie within the germ-wall at the posterior margin of the pellucid 

 area. 



This is illustrated by Figs. 30 and 31, representing a longi- 

 tudinal section and a reconstruction of a blastoderm three hours 

 before laying. Considering the reconstruction first, it will be 

 noted that the lip of the blastopore, R, now lies within the blasto- 

 derm at the posterior margin of the pellucid area. The greater 

 portion of the pellucid area is now two-layered owing to the 

 continued expansion of the entoderm E, which has met and 

 united with the germ-wall at .the sides. The section (Fig. 30) 

 passes longitudinally through the center of the blastoderm. The 

 mass of cells at D represents the original lip of the blastopore. 

 It is continuous with the germ-wall behind and with the ento- 

 derm in front. The latter is not a continuous layer (Fig. 30 A), 

 and the cells are not coherent. It is probable that the extension 

 of the entoderm is due largely to independent migration of the 

 cells. Subsequently the entoderm cells unite to form a coherent 

 layer of flattened cells. (See Chap. IV.) 



In some cases the closure of the blastopore takes place in 

 such a way as to produce a marginal notch, which is referred 

 to again in connection with the primitive streak (Chap. IV). 



