426 
BULLETIN OF THE UNITED STATES FISH COMMISSION. 
Up to this time the marginal cells have had boundaries quite as distinct as any 
other cells of the blastoderm. At the end of the seventh segmentation (fig. 44) some of 
the marginal cells have the lower margin ill defined and continuous with the yolk. This 
is never the case in more central cells. The nucleus is also slightly more refringent 
in these marginal than in the neighboring cells. At the end of the eighth segmentation 
(fig. 45) there is still a distinct dorsal wall to the periblast cell, but its nucleus is now 
much larger than that of the surrounding cells. At the end of the ninth segmen- 
tation the periblast consists of a few large, refringent nuclei imbedded in protoplasm, 
which is restricted to the immediate neighborhood of the blastoderm (figs. 47-49). 
At this time the outermost layer of the blastoderm has begun to creep over the yolk, 
so that the periblast cells are partly covered exteriorly by the rim of the blastoderm 
formed by the outermost cells. Dorsally the periblast cells are covered by the row of 
cells just within this outer projecting series. The periblast does not extend beneath 
the central cells of the blastoderm and the nuclei do not reach this region till later, 
after the nuclei are all that remains of the periblast. So much has been written con- 
cerning the fate of the periblast that a detailed account of it in Cymatogaster , where it 
has been reduced to its simplest formula, will follow (p. 437). 
After the seventh segmentation and up to the time of the formation of the first 
protovertebra; the development of Cymatogaster shows little in common with other 
fishes, and an egg of this species found floating during this period would scarcely be 
recognized as belonging to a teleost. The rapidity characterizing the formation of 
the periblast characterizes the whole of this development and the difference of the 
blastoderm at the beginning and at the end of any segmentation is sometimes con- 
siderable. The age of each of these stages was determined by the number of nuclei in 
the blastoderm. While the method of counting nuclei is probably not as satisfactory 
as observing the successive segmentations in the same egg, this method is almost as 
exact, because the number of nuclei present must be doubled or diminished by half in 
order to change the serial number of the segmentation under consideration. 
During the ninth segmentation the marginal series of cells of the blastoderm creep 
over the yolk. At the same time they become flattened (fig. 27). Near the end of the 
tenth segmentation about three series of cells (only one cell deep) have crept over the 
yolk and cover it from the blastoderm, which has scarcely changed shape, to the 
yolk nucleus, the margins of which have been extended to meet this layer of thin cells. 
The yolk at the end of the tenth segmentation is therefore entirely inclosed (fig. 29). 
During the eleventh segmentation the yolk sinks into the blastoderm, or, in other 
words, the blastoderm spreads over the yolk; but the former expression is more apt, 
since the process in no way resembles the epibolic growth of the blastoderm over the 
yolk in other teleosts. In other fishes the inclosing of the yolk is accomplished by 
the growth of the gastrula margin, while in Cymatogaster it is accomplished before 
there is any sign of gastrulation. The inclosing of the yolk by the blastoderm during 
the eleventh segmentation corresponds rather to the spreading of the blastoderm in 
pelagic eggs just before the beginning of the marginal infolding. The yolk in 
Cymatogaster appears to sink into the blastoderm on account of its small size. At 
the end of the eleventh segmentation the cells of the blastoderm are not yet divided 
into two layers and the yolk nucleus is still exposed. (Figs. 32 to 34.) A noteworthy 
feature is that near the end of the tenth segmentation the outermost layer of the 
blastoderm consists of flat cells, which contrast strikingly with the remaining more 
rounded cells. This differentiation does not exist except at the margin before the 
