Embryology: Stage V Back

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Stage V of the embryonic development is marked by teh appearance of several new organs, mainly in the field of teh nerveous system and the sensory and respirational organs.
The size of the embryos of both Pila globosa and Marisa cornuarietis have increased to a length of about 340µm. The shape of the embryo has become more pear-like as the anterior part (head area) is more spherical, while teh posterior side is elonguated.
The asymmetry of the embryo is further reduced and the shift of the shell gland and rudimental shell plate to the left side has progressed. The first real shell material is deposited in the center of the shell plate. The shell plate itself is considerably enlarged during this stage.
At the ventral side, the foot is enlarghed and points to the ventral side and becomes V-shaped. The cells of the pedal plat, at the ventral side of the foot, increase in size and become aligned along the foot in medial-longitudinal direction.
The velum at the dorsal side of the embryo, is enlarged at the front side and forms two velar lobes, one at each side of the head. Ranjah (1942) reported this formation to take place in an earlier stage in Pila globosa.
New organs start to develop like the ctenidium and the osphradium that become conspicuous as thickened oval rudiments in the ectoderm, located at the dorsal side of the visceral sac. The ctenidium will function as the snail gill in the future, while the osphradium becomes an important chemo-sensory organ in the snail.
Besides the osphradium, another sensory organs develops durings this stage: the statocyst. These equilibrium sensors become visible as small thickenings in the ectoderm, laterally on each side of the embryo, ventral from the endodermal sac. In a later stage these thickenings will invaginate and form vesicles inside that relocate inside the snail body.
Cells of the ectoderm start to differentiate into nerveous cell. The first ganglia become visible in form of the cerebral, pedal, pleural and instestinal ganglia. All these structures are located in the ectoderm in thise stage of development.
Inside the embryo, the cavity of the endodermal sac differentiates further, with as result a better visible anterior stomach cavity with a thick wall and thin walled rudimental instines at the posterior side. The latter (intestines) is a sort of tube, still ends blindly in the Marisa embryo, whereas the Pila globosa embryo is reported to have an anal opening at the end of the intestines from stage I.
The perdicardium is a fairly large sac in stage V, and moves backward in the embryo, until is lies behand the endodermal sac. Inside the pericard (heart sac), the heart evolves into a bi-lobed structure due to invaginations, to form the auricle and the ventricle.
The shape of the rudimental kidney has changed and the whole organ is now tube-like, connected with the ureter. The ureter itself has elonguated into a long tube, that bends forward. The opening of the ureter (the renal vestibule) has shifted below the pericard. In letaral view, the ureter is now U-shaped.
The first rudimental blood vessels and sinusses apear in the mebryo as irregular spaces within the mesenchym.



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