Cuap. II. FOLDINGS OF THE EMBRYONIC DISC. 537 
disc (Pl. 11, fig. 6a, and Pl. 9e, fig. 2) shows at once the nature of this change, 
and at the same time discloses a thickening at the part (Pl. 9e, fig. 2, a; 
Pl. 11, fig. 6a, «@,) where the depression occurs, and also that the subsidiary 
layer (Pl. 9e, fig. 2, o') follows this depression. By this time the yolk mass has 
begun to recede from this spot, and is replaced by the albumen which has fil- 
trated through the several walls and layers around the yolk. As yet, however, 
the albumen (PI. 11, fig. la; Pl. 9e, fig. 2, a/) occupies but a small segment of 
the yolk sac. The next older phase has brought the wings of the depression 
nearer to one another, so that the central part of the latter is bounded, on the 
side next to the disc, by what resembles a cone confronted by the approximated 
homer (Pl 11, fig. 2, By this 
time, too, the opposite end (PI. 11, fig. 2, a) of the disc has become considerably 
depressed, yet not like the other end, (a',) but simply by curving down, while keep- 
ing its contour outwardly arched; the right and left sides also are slightly folded in, 
a') which bounded the formerly lunate hollow. 
in a downward direction, carrying with them a broad strip of the neighboring 
space, and thus forming a deep annular depression all round (PI. 11, fig. 2, ¢).' 
1 Among the eggs which were retained in the 
oviduct by a female in confinement beyond the usual 
time of laying, we have found some remarkable in- 
stances of monstrosity. 
We will first mention one found in the egg of 
Malacoclemmys palustris. As the embryo normally 
develops, the caudal hood, as just stated, commences to 
form almost immediately after the cephalic hood, but in 
the instance before us (PI. 11, fig. 7, a°, 7a, a, a°, 7b, 
@, a’) the head is strongly bent upon itself, whilst the 
caudal end (fig. 7b, a*) is not folded in the least. The 
back of the embryo is also more arched (fig. 7b) than 
in the normal state. 
Another instance, of much more extreme disparity 
between the two ends of the embryo, was found in the 
ege of Ozotheca odorata. The embryo, instead of hay- 
ing its normal round or broadly oval form, suddenly 
narrows behind to half its anterior width, and then 
terminates in a rounded end (PI. 11, fig. 9, 9a, 9b). 
Here again the cephalic hood alone has developed, 
and that, too, far beyond the bounds of normality. 
After having bent upon itself as is usual, a portion of 
the head along the axial line has continued to push 
still further back in the form of a blind sae, till it has 
reached the posterior end of the embryo (PI. 11, fig. 
9, 9b). Seen from above, (fig. 9,) the blind sac ap- 
68 
pears broader behind than at its mouth, and in a lon- 
gitudinal section (fig. 9b) we see that it is quite flat, 
and proceeds in a straight line from the head to the 
tail, and also that the back of the embryo is much 
We have 
also made a cross section of this embryo, just behind 
more arched than in the preceding case. 
the head, (PI. 11, fig. 9a,) in order to display the trans- 
verse arch of its back and the flatness of the whole 
width of the blind sac, and the manner in which its 
mouth expands sideways and joins the more periphe- 
ric part of the embryonic disc. It will be noticed in 
this transverse section (fig. 9a) that the sides of the em- 
bryo are rather suddenly bent downwards; but at the 
caudal end there is no folding, (fig. 9), notwithstanding 
the highly developed character of the cephalic hood. 
In another instance, (PI. 11, fig. 8, 8a,) the embryo 
of this same species exhibits the normal oval shape, 
but otherwise resembles the last in the mode of its 
development. The blind sac, however, is cylindrical, 
and does not reach more than three quarters of the 
way toward the caudal end (fig. 8). At the mouth 
of the blind sac there is a furrow on its upper side, 
(fig. 8a,) which might be mistaken for the primitive 
furrow, but it is probably a longitudinal fold. 
In another egg, of Malacoclemmys palustris, the 
embryo appears, at the first glance, perfectly normal 
