HEAD-FOLD TO TWELVE SOMITES 129 



Centrally to the zone of junction we have the differentiated 

 ectoderm and germ-wall sharply separated from the yolk by the 

 perilecithal space. The ectoderm of the inner zone of the vitelline 

 area requires no extended notice ; it consists at this time of a sin- 

 gle layer of flattened cells. The germ-wall next to the zone of 

 junction consists of two or three layers of large, more or less 

 rounded, cells with definite boundaries, each of which contains 

 one or more yolk-spheres and smaller yolk-granules (Fig. 68 E). 

 We may say roughly that whereas in the zone of junction we 

 have cells in the yolk, in the vitelline area we have yolk in the 

 cells. This mav indicate sufficientlv the wav in which a several 

 layered epithelium becomes differentiated from the zone of junc- 

 tion. As this epithelium is traced centrally we find usually a 

 short distance from the zone of junction a thinner area (Fig. 

 68 D), and beyond this again the several layers of cells even 

 more laden with yolk-spheres and granules than previously; so 

 that it would appear that these cells may actively engulf yolk- 

 granules. At the margin of the vascular area the entoderm be- 

 comes one-layered, and is composed of columnar cells with swollen 

 free margins turned towards the yolk and still containing some 

 yolk-granules and spheres (Fig. 68 C). At the margin of the 

 pellucid area there is a rather sudden transition to the flat ento- 

 dermal epithelium characteristic of this area. 



