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 KE, 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). 
