258 
BULLETIN OP THE UNITED STATES FISH COMMISSION. 
seen iu Fig. 5, after segmentation is completed. About this time a segmentation : 
cavity appears within the egg, roofed over by the smaller segmentation spheres of the 
dark pole, while it is inclosed below by the large spheres of the lighter pole, which 
comprises the yolk. Just at this point there is an inconsistency in the account given 
by Salensky which it is hard to reconcile with the very large persistent yolk of the 
later stages. The large segmentation spheres seem to have their segmentation furrows 
obliterated at a later stage; at any rate the yolk is said to be inclosed by the walls of 
the primitive entoderm of the intestine, after the segmentation of the yolk substance 
has become less obvious than at first, and after its nucleated segments had become 
pretty numerous. If the inclusion of the yolk cells by the intestine actually occurs as ' 
described by Salensky, the process is without a parallel among the vertebrates, the | 
nearest approach being the mode of yolk absorption lately described by P. and F. 
Sarasin as occurring iu the embryos of IcMhyophis glutinosus, one of J;he footless, worm- 
like batrachia of Ceylon. 
The further development of the germinal area at the dark pole of the egg presents : 
many analogies to that of the osseous fishes, especially as respects the manner in ! 
which the yolk is finally covered in and completely inclosed. Upon comparing Fig. i 
6 with the stage represented in Fig. 5, it will be seen that the upper pole is now I 
covered by a cap of cells which leave only the lower coarser yolk cells exposed. This 1 ' 
cap has resulted from the more rapid segmentation of the small cells shown at the | 
upper pole of Fig. 5. These smaller cells of Fig. 5 have in fact, in Fig. 6, become so i 
small by repeated division that the artist has found it impracticable to represent them l|| 
according to the scale of enlargement adopted in the last-mentioned figure. The cap 
of cells has grown all around its margin in such a way that it has gradually extended L 
over the yolk. At one side of the cap the rudimentary embryo is visible as a light j;! 
band rounded at the upper end. A linear depression runs through the middle of the 
rudiment of the embryo ; this represents the center of the so-called medullary plate, U 
and is also the middle of the medullary groove to be folded in later when the 'ff 
margins of the medullary plate are turned upward and fuse immediately over the | i 
former to form the medullary canal or passage way through the primitive spinal cord, |.'i 
which is developed in the way above described. . l i 
In the next stage of development, which is reached in about twenty-four hours after 1 1 1 
impregnation, we have the appearance presented in Fig. 7. The only portion of the ii;j 
mass of yolk cells which now remains exposed is indicated by the little rounded, nearly Ij! 
black area, at the lower pole of the figure. The yolk is now completely inclosed, u! 
except this small portion, by the continued growth of the lower edge of the cap repre- r| 
sented in Fig. 6. This small area of yolk still exposed is the yolk blastopore or || 
prostoma. |i] 
At this time the rudiment of the embryo becomes widened in front, as shown in 
Fig. 7, so that the medullary plate, anteriorly, becomes expanded like the broad end |‘ 
of a spatula. This widened end of the medullary plate is the rudiment of the future > 
sturgeon’s brain, and behind this widened part, the edges of the medullary folds | - 
become more distinjctly evident, since the development of the spinal cord of which 
they form part is now more advanced than in the preceding stage. The embryonic h 
area at this time presents some characteristic features of coloration in the common f 
sturgeon. The area immediately around the embryo is light-colored; some little j 
distance from it there is a darker band of color. This dark band is oblong, with the h 
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