428 
BULLETIN OF THE UNITED STATES FISH COMMISSION. 
tip of the embryonic shield is separated from the inner margin of the anterior part of 
the embryonic ring by but a very narrow space, through which are scattered cells; in 
other words, there is a very close approach to a diplastic gastrula. It is not at all 
improbable that in this genus eggs may sometimes occur in which the lower layer is 
quite continuous, and that a two-layered gastrula is formed. This condition, however, 
lasts but a very short time. With the epibolic growth the lower layer becomes more 
and more incomplete. The two-layered condition of Cymatogaster seems to be homolo- 
gous to this early two-layered condition of Stolephorus. 
Gastrulation. — On account of the difficulty of orientation of the stages during 
gastrulation, and because the contents of the zona soon fill the entire space within 
it, and some structures are invariably injured, owing to the necessity of cutting 
the zona, the process of gastrulation has not become very lucid. It will be best, 
therefore, to describe in detail the few favorable series of sections. 
To repeat the description of the stage during the eleventh segmentation or just 
before the beginning of gastrulation : An egg, near the end of the eleventh segmenta- 
tion (containing about 1,700 cells) was cut into twenty sections, of which, sections 
3 to 15 cut through the yolk. The sections run somewhat obliquely to a meridian 
plane, as is indicated in the diagram, fig. 31. A section (the ninth) through near 
the middle of the yolk (the sixth section cutting through the yolk) is represented in 
fig. 33. As will be seen by the figure and by the diagram, the yolk has sunk into 
the blastoderm. At the ectodermic pole the blastoderm is about five cells deep. It 
becomes shallower towards the entodermic pole and gives out altogether at the yolk 
nucleus which has spread to some extent over the yolk. The yolk is, therefore, 
entirely inclosed, and the margins of the blastoderm are separated only by the yolk 
nucleus. There is no indication of a division into entoderm and ectoderm; in other 
words, gastrulation has not yet begun. There are present about 14 periblast nuclei. 
The breathing chamber has not been greatly reduced. The height of the egg is -054 mm., 
the width - 064 mm., the diameter of the zona *073 mm. 
The next stage satisfactorily made out is of several eggs during the twelfth 
segmentation. There are, however, two stages of development, which, according to 
the estimate of cells, belong to this segmentation. Although the two stages differ 
considerably, it is difficult to state at a glance which is the older. In the one case 
(fig. 38) the blastopore is still open, and the yolk nucleus forms a plug which fills it. 
The ectoderm and entoderm are well separated, while the cells have become nearly 
evenly distributed over the yolk. In the other (fig. 35) the shape is much more like 
that of the eleventh segmentation; the ectoderm and entoderm are scarcely separate 
and the cells are still heaped up at the ectodermic part of the yolk, but the blastopore 
is much smaller. 
Thus, while the shape and relation of ectoderm to entoderm indicate the latter 
(fig. 35) to be less advanced, the large blastopore points to the former (fig. 38) as the 
earlier stage. Everything considered, it is evident that fig. 35 represents the younger 
stage. The two stages are very close together and also near fig. 41. The ovary from 
which the egg represented in fig. 35 was taken contained several different stages, and 
in fact one egg in an earlier stage than the one represented in fig. 33 and another in 
about the stage represented by fig. 38. 
In fig. 35 the outermost layer of cells is quite distinct from the others. The 
cells are mostly flat, quite small and stain (with Grenadier’s alcoholic borax carmine) 
