210 DEVELOPMENT OF FISHES 
flattened, but extends to the marginal cells of the germ 
disc; in Fig. 272 its roof consists of two tiers of blasto- 
meres, its floor a thin film of the unsegmented substance 
of the germ; the marginal blastomeres are continuous 
with both roof and floor of the cavity, and are produced 
into a thin film which passes downward, around the sides 
of the yolk. In Fig. 273 the segmentation cavity is still 
further flattened; its roof is now a dome-shaped mass of 
blastomeres ; the marginal cells have multiplied, and their 
nuclei are seen in the layer of the germ, P, below the 
plane of the segmentation cavity. These are seen at WP 
in the surface view of the marginal cells of this stage 
(Fig. 274); they are separated by cell walls only at the 
sides ; below they are continuous in the superficial down- 
reaching layer of the germ. The marginal cells, JP, 
shortly lose all traces of having been separate; their 
nuclei, by continued division, spread into the layer of germ 
flooring the segmentation cavity, and into the delicate film 
of germ which now surrounds the entire yolk. Thus is 
formed the ferzb/ast of teleostean development, which from 
this point onward is to separate the embryo from the yolk; 
it is clearly the specialized inner part of the germ, which, 
becoming fluid-like, loses its cell walls, although retaining 
and multiplying its nuclei. It would accordingly corre- 
spond to that portion of the germ of the sturgeon in Fig. 
253 which lies below the plane of the segmentation cavity, 
and which extends downward at the sides of the yolk; in 
this case, however, the surface outlines of the cells have 
not been lost. It will be seen from later figures (Figs. 
278-282) that the periblast, P, comes into intimate rela- 
tions with the growing embryo; it lies directly against 
it, and appears to receive cell increments from it at various 
regions; on the other hand, the nuclei of the periblast, 
