Die Embryonalentwicklung der Paradiesschnecke ... - TOBIAS-lib

Kapitel 2
has become flattened laterally. The rudimentary shell gland enlarges and has
shifted to the left of the visceral sac. Its central part, the mantle anlage, has
grown excessively and, also according to Demian and Yousif (1973a), induces
an anticlockwise rotation of the visceral sac (by an extremely imbalanced
growth of both lateral sides) and the formation of the mantle.
In contrast, embryos exposed to platinum correspond to Stage V or VI after
3 days of development (Fig. 2C). The prototroch is still clearly visible, and
the visceral sac is only slightly enlarged. The rudimentary shell gland still
forms a depression on the visceral sac, and the mantle edge is not enlarged
as well. Figure 2B,D depicts histological sections of the visceral sac showing
the similarity in the anatomy of differently treated embryos.
Embryos both from the control (Fig. 2B) and the platinum group (Fig.
2D) resemble normally developing M. cornuarietis embryos in Demian and
Yousif ’s Stage VI. In both groups, the rudimentary shell gland has enlarged
in comparison to earlier stages. Its center, the mantle anlage, has bulged up
and is covered by a thin layer, the protoconch, or larval shell. Its peripheral
cells have formed a thickened circle that is expected to become the adult’s
shell gland in the conventional body plan. In this stage, the mantle edge is
a protruding fold that encircles the shell gland.
Four- and four-and-a-half-day-old embryos
At this stage of development, the embryo from the water control (Fig. 3A,B)
can easily be recognized as a snail. The tentacles have started to be formed,
and the mouth has developed. The foot has reached its final shape and
position. Between foot and shell, the operculum has been formed. The shell
covers almost all the visceral sac. The mantle edge, a thick ridge, limits the
extension of the shell to the anterior.
Pt-exposed embryos (Fig. 3C,D) are clearly lagging behind in their development
compared to controls. They resemble embryos in Stage VI or VII
except for features of shell formation. In Stage VI, when the rudimentary
shell gland usually enlarges, bulges outward, and thickens at the periphery
(shell gland of the adult), platinum arrests outgrowth completely or right af-
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