a0 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 germ-wall. 
The central as well as the marginal periblast contributes to the 
blastoderm, but the former appears to be rapidly used up. The 
marginal periblast 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. 
Fia. 23. — Posterior end of 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, Syneytial 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-sae entoderm which becomes 
continuous with the embryonic entoderm secondarily. The term 
verm-wall is usually applied to the primordium of the yolk-sae 
