Die Embryonalentwicklung der Paradiesschnecke ... - TOBIAS-lib

Kapitel 2
through a control embryo is shown. Until hatching, the embryonic development
of controls is completed (Demian and Yousif, 1973a). Also, the embryos
from the platinum exposure group have developed further (Fig. 7C). The
developmental state of tentacles, foot, and ctenidium now correspond well
to control embryos in Stage XI, the stage that is usually reached in controls
after 6 days postfertilization. The ctenidium has elongated and added ctenidial
leaflets, but it is still located on the left side of the visceral sac. Neither
mantle edge nor shell gland nor the mantle anlage have grown craniad but
remain to be located on the ventral side of the visceral sac. This can also be
seen in Figure 7D, which shows a transverse section through the visceral sac
of a Pt- exposed embryo. The shell-secreting tissue (arrows in Fig. 7D) is
positioned in a cavity on the ventral side of the visceral sac right above the
foot.
The rotation of the visceropallium
Figure 8A shows an embryo from the control group 3-day postfertilization.
The visceral sac has rotated approximately 20 ◦ anticlockwise (angle between
dashed lines 1 and 2 in Fig. 8A). The arrow indicates the direction in which
the distal part of the mantle edge and the shell gland will grow as a result of
the growth of the mantle anlage and the outer mantle epithelium. In contrast
to that, the 4-day-old embryo from the platinum group (Fig. 8) does not
show any rotation at all. At the age of 5 days (Fig. 8C), the rotation
has been completed in the control embryo, whereas in the embryo from the
platinum group the visceral sac still lays on the same axis as head and foot
(Fig. 8D). Figure 8D shows that mantle edge, shell gland, and mantle anlage
have been shifted from their initial left lateral position to the ventral side of
the visceral sac indicating that, instead of a horizontal rotation by 180 ◦ , a
vertical rotation of the visceral sac by 90 ◦ has taken place.
Figure 8E,F displays the positions of intestine and anus in an adult shellless
snail (Fig. 8E, anus with feces) and in a 7-day old, partly dissected
embryo from the platinum exposure group (Fig. 8F). A comparison with the
description of the development of the alimentary tract by Demian and Yousif
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