Cr 
wo 
Oo 
EMBRYOLOGY OF THE TURTLE. Part III. 
the organs within the embryonic disc begins (PI. 11, fig. 1, a') before its outlines 
are well marked by the deepening of the circular furrow. However, we may 
mention that it is usually the fact that the embryonic dise becomes sharply 
bounded before any change comes over it. 
By the time the embryonic disc has become well defined, (Pl. 10, fig. 12-15,) 
there is already a difference noticeable in the nature of the cellular components of 
its upper and of its lower side. Those above are by far more uniform in size, 
and smaller (Pl. 9a, fi 
layer (Pl. 9e, fig. 1, a) of so smooth a surface as to give to the embryonic disc 
g. 34, a) than those below; they form a thin, uninterrupted 
that polished aspect mentioned before, while those below are coarser and darker, (PI. 
Ya, fig. 54, 6,) still evincing a cumulated arrangement, and, in some instances, even 
restrained by the parental envelope." These lower cells form by far the thicker 
layer (Pl. 9e, fig. 1, 0'). The upper layer (a) is continued over the whole sur 
face of the yolk; but here it is not quite so thin as on the embryonic disc. 
The lower layer (o') follows the upper, (a@,) but is not so distinct from the sub- 
jacent more mobile yolk mass (yk); yet it is sufficiently separated from the yolk 
to be easily recognizable, even though it forms an intermediate stage between the 
two. The thinner or upper layer (a) more properly deserves the name of ger- 
minal layer, and the other, the thicker or lower, may be considered as a_subsid- 
iary layer, (0',) the upper surface of which is constantly furnishing supplies to 
the thickness of the upper layer, (a,) and is continually added to from below, (y/,) 
for a certain length of time, which varies according to the part of the whole 
developmg mass to which it belongs. We have mentioned before in passing, 
that, in the vascular area, there is a constant addition to this layer, even up to 
the end of the period of incubation. 
The Annios. We have already noticed the initial steps toward the formation 
of the amnios, when pointing out the cireular furrow which bounds the embry- 
onic disc.” "This furrow ‘(PI “Qe, fig. “1,-¢; Pl WOj-fig, 12, 13, 14) @, 95) isemet 
formed by a scooping out of the thickness of the germinal layer. It is, on the 
contrary, the result of an actual depression of its whole thickness (PI. 9e, fig. 1, 
c, ¢); so that, if viewed from below, that region would appear to have a circular 
semicylindrical ring raised upon its surface. This is the first indication of the 
foldme of the germinal layer to form the amnios. 
But soon this uniformity yields to a different tendency. The edge of the 
embryonic dise becomes suddenly depressed at one point, so that, viewed from 
above, it appears. as if a narrow segment of a circle had been cut away from 
its sides (PL 11, fig. 1, a’, 6, a’). A longitudinal section of the embryonic 
1 See p. 519-522, and PI. 9a, fig. 7, 9, and 34. 
