Most of the pictures were taken through a 10x objective and a 10x ocular , with a resolution of 640 x 480 px. (if there is no contrary indication in the picture caption). The 480 px side of the pictures measure exactly 1000 microns. Darkfield and various Rheimberg filters were used to give more contrast and definition to the pictures.
EMBRYOS OF PHYSELLA sp.
Almost all the freshwater gastropods put their eggs in jelly-like masses adhered to different surfaces (stones, leaves and stems of submerged plants). At the aquarium they choose generally the glass walls where they are easily visible like little discs, in some species, or with a horseshoe shape like in the one shown below.
The picture shows a gelatinous ootheca that contains 90
which allows us to easily follow the embryo development. The eggs recently
a cytoplasm full of yolk and with a big nucleus. In just a short time
egg divides into two cells, soon into four,
8, 16 cells and form at the end a morula,
that is to say, a spherical
and compact mass of more or less spherical cells (of course the name comes from
the similarity of this embryo with the fruit of the Morus sps. trees, the mulberries).
From now on, it is more
difficult to follow the detailed development, because the most
changes happen in the interior of the embryo, and it would need
the aid of
a histologist to identify them.
An interesting detail of the development of the mollusks is that they start as “right” and symmetrical, but soon begins a process of asymmetrization and torsion of the visceral mass, at the same time that this is being covered by the segregated calcareous shell.
The torsion causes the atrophy of the organs in the left side of the body, if torsion is to the right, or on the right side if it is to the left.
When growing, the shell follows the helical torsion process generating the gastropods characteristic spiral. As is logical, with the growth of each successive turn the shell constructs a center axis the “columella”. A beautiful picture of this structure is seen in fig. 33.
These are two pictures of other more developed embryos.
In just a short time the little snails begin to move within the egg, as the shown in the following animations.
If you look closely at some of the more sluggish embryos, you can see the beat of the heart of the growing embryo.
Soon they break through the egg’s shell and leave it. The youthful individuals are beautiful photogenic subjects.
Peter Abraham .- Stereo X-rays of sea shells
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