2IO 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 MP 

 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, MP, 

 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 periblast 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, 



