|Embryology: Stage VII|
Stage 7 of the embryonic development is characterized with the start of the body torsion, which continues and finally completes at the end of stage 10. At the start of stage 7, the Marisa cornuarietis embryo is about 570µm in length, while the Pila globosa embryo measures around 615µm length.
During stage 7, the outlines of various organs become visible, while the embryo
itself is transparant. The visceral sac at the posterior (back) side of the
embryo is further enlarged and becomes laterally compressed. Also the first
signs of torsion appears as the visceral sac rotates counter-clockwise towards
the head area.
The shell gland, which is located at the left side of the compressed visceral sac, enlarges and becomes almost completely flattened. In the center of the circular shell gland, the rudimental shell plate develops in what will become the future shell.
The foot of the embryo becomes gradually elonguated and teh broad, flattened sole is now used by the embryo to creep over the inner surface of the egg.
Inside the mebryo, the intestinal tract develops further and with this, the radular sac becomes visible in the head of the embryo. More in the middle of the embryo, the stomach increases in size and the left gastric streak becomes visible at the left side of the stomach. In the latter stages, this gastric streak will develop into the pancrea-hepatic vestibule of the snail. The rudimental instestines shift to the right side with the ongoing torsion of the visceral sac. However, the intestines are still closed in the Marisa cornuarietis embryo, while being open and connected to the anus in the Pila globosa embryo.
The ctenidium and the osphradium move to the front of the embryo and become thicker and larger.
On the neural side of development, various ganglia differentiate and the effects of torsion become conspicuous in the form of an assymetric nerveous system. The 2 pedal ganglia fuse with the 2 pleural ganglia, forming 1 ganglia at each side of the body. The 2 intestinal ganglia rotate around the stomach, one becomes the sub-intestinal ganglion, the other forms the supra-intestinal ganglion. Further integration of the various ganglia starts with the formation of commissures and connectives.
The statocyst, which arised as invaginations during stage 5, now develop into vescicles, seperate from the epidermis.
The pericardium has increased to around 100µm in diameter, and now lies dorsally to the rudimental intestines. Inside the pericardium one can see the heart chambers, which are completely seperated into an auricle and a ventricle. These heart chambers show the first clear, but still irregular, contractions. Towards stage 10, these contractions will become more regular.
The kidney, now oval in shape, starts to shift to the dorsal side. The U-shaped ureter is flattening in the sense that both arms of the U are lying side-to-side.
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