520 EMBRYOLOGY OF THE TURTLE. Part III. 
round the germ being already present as an indication of the incipient plica- 
tion of the amniotic membrane; and here the greater part of the mesoblasts are 
still further and more minutely divided, (PI. 9a, fig. 4, 5, 5a, 6, 8,) and reduced 
to that size (fig. 34, a) which they exhibit when the organs have begun to 
mark out their boundaries. 
But, let us return and trace more critically the changes through which the 
yolk cells pass, in order to reach that condition im which they are found when 
they have become components of the nascent embryo. By the time that the 
segmentation of the yolk has commenced, not only the cells in the region of the 
embryonic area, but those all over the surface of the yolk close to the vitellme 
envelope, have multiplied their mesoblasts to an innumerable number. In this 
state, they may be recognized as a very light yellowish white layer, which — when 
the egg is rolled in various directions, the more interior yolk thus falling to 
what becomes, in succession, the lower side, and this layer is left more exposed — 
resembles a very fragile, sedimentary deposit agaimst the yolk sac, fallimg away 
in flakes upon the least flexure or disturbance of its smooth, crust-like arch. 
Owing to the rapid formation of this layer, and the quickly succeeding ultimate 
changes in the cells, the latter seem to burst almost in an instant, and leave 
their mesoblasts arranged in heaps, (fig. 55a,) side by side, thus forming the brittle 
stratum above mentioned. The superficial ectoblastic cells of this layer discharge 
their mesoblasts so early before the self-division of the latter has been completed, 
that it is next to impossible to find among them ectoblasts still embracing their 
progeny ; but this may be accomplished in regard to those more deeply seated, 
especially next to the inner surface of the stratum. We will first make a special 
mention of these latter, and then return to the former to trace their progress in 
completing the stage of their fissiparous multiplication, and the connection of the 
same with the building up of the embryo. 
By cutting out an embryonic disc and laying it upon its back in a watch-glass 
containing albumen from the same egg, it is very easy to select whatever portions 
are needed from this body for this purpose; the substance hanging together so lightly 
that a few cells may be taken up on the poimt of a knife and laid upon a glass 
slide for examination, or, for those most superficial, the microscope may be brought 
to bear directly on them in situ. Of course, in the latter case, a strong, concen- 
trated light from above is necessary, on account of the opacity of the embryonic 
dise, which precludes the possibility of using transmitted illumination. The inner- 
most cells, the ectoblasts, of the germinal layer and of the embryonic dise,' are still 
1 Tf, however, there are those who still incline then they must, even upon this supposition alone, 
to believe that these cells are genuine segment balls, admit also, without reservation, that segmentation 
