50 THE DEVELOPMENT OF THE CHICK 



a ring around the blastoderm. It persists during the expansion 

 of the blastoderm over the surface of the yolk. 



The blastoderm now begins to expand, owing largely, at first, 

 to additions of cells to its margin cut off from the periblast. The 

 central as well as the marginal periblast contributes to the blas- 

 toderm, but the former appears to be rapidly used up. The mar- 

 ginal periblast, which is commonly called the germ-wall from 

 this stage, on the other hand grows at its periphery while it adds 

 cells to the blastoderm centrally, and thus it moves out in the 

 white yolk, building up the margin of the blastoderm at the same 

 time. The original group of central cells appears to correspond 

 approximately to the pellucid area; the additions from the germ- 

 wall would thus constitute the opaque area. 



4 



Fig. 23. — Posterior end cf a longitudinal section through the blastoderm 

 of a pigeon's egg about 25 hours after fertilization (8.50 p.m.). (After 



Blount.) 

 1, Nests of periblast nuclei. 2, Periblast nucleus in marginal position. 

 3, Syncytial mass derived presumably from the periblast, in process of or- 

 ganization into cells. 4, Vacuoles. 



Some phases of these processes are illustrated in Figs. 23 and 

 24. In the vertical section, Fig. 23, the surface of the germ- 

 wall next the blastoderm is indented as though for the formation 

 of superficial cells. Along the steep central margin of the germ- 

 wall groups of cells are apparently being cut off and added to 

 the cellular blastoderm. In the horizontal section, Fig. 24, the 

 process of cellularization at the central margin of the germ-wall 

 is apparently proceeding rapidly. 



The superficial cells thus added to the margin of the cellular 

 blastoderm become continuous with the ectoderm, and the 

 deeper layers later form the yolk-sac entoderm which becomes 

 continuous with the embryonic entoderm secondarily. We can 



