i 36 Embryology and Growth of Fishes 



greatly flattened, but extends to the marginal cells of the germ- 

 disk ; its roof consists of two tiers of blastomeres, its floor of a 

 thin film of the unsegmented substance of the germ; the mar- 

 ginal 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. Later the segmenta- 

 tion 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, below the 

 plane of the segmentation cavity. These are seen in the sur- 

 face view of the marginal cells of this stage ; they are separated 

 by cell boundaries only at the sides ; below they are continuous 

 in the superficial down -reaching layer of the germ. The mar- 

 ginal cells 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 the Teleost development, which from this point on- 

 ward 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. Later the periblast comes into intimate relations 

 with the growing embryo; it lies directly against it, and ap- 

 pears to receive cell increments from it at various regions; on 

 the other hand, the nuclei of the periblast, from their intimate 

 relations with the yolk, are supposed to subserve some func- 

 tion in its assimilation. 



Aside from the question of periblast, the growth of the 

 blastoderm appears not unlike that of the sturgeon. From 

 the blastula stage to that of the early gastrula, the changes 

 have been but slight; the blastoderm has greatly flattened out 

 as its margins grow downward, leaving the segmentation cavity 

 apparent. The rim of the blastoderm has become thickened 

 as the ' germ-ring' ; and immediately in front of the dorsal lip 

 of the blastopore its thickening marks the appearance of the 

 embryo. The germ-ring continues to grow downward, and 

 shows more prominently the outline of the embryo; this now 

 terminates at the head region ; while on either side of this point 

 spreads out tailward on either side the indefinite layer of out- 



