Embryology - Stage VI

Embryology: Stage VI

In stage 6 the embryo has grown considerably. The Marisa cornuarietis embryo even surpasses the Pila globosa embryo in this stage (450µm versus 390µm). The main increase in size is lengthwise, whic results in a more laterally compresses embryo.
While the symmetry of the embryo already vanished in earlier stages, the head area (anterior side) and the visceral sac at the posterior side are still on one straight line. In other words, the torsion of the body is still abundant.
The yellow apperance of the embryo has diminished, except for the alimentary canal, which is still filled with yellow yolk (albumen).
The foot is further enlarged and gets more conical in shape, with a flattening at the antero-ventral side. The latter part of the foot will develop into foot sole, and the pedal cell plate is reduced to a small ridge at the median line of the foot sole. Due to the lateral compression between the foot and the velum, the foot itself is much easier to distinguish from the head than it did in the previous stages.
At the posterior side of the embryo, the visceral sac becomes enlarged and laterally compressed. At the left side of the now flattened visceral sac, the shell gland with the rudimental shell plate in its center, is further enlarged. The shell plate starts to bulge outwards, making the shap cup-like. In the next stages, this cup formation will continue, ultimately engulfing the visceral sac and other organs to form the shell.
At the anterior side, the differentiation of the mouth region progresses and the mouth becomes partially encircled by rudimental lips. Inside the mouth region, the radular sac enlarges, which defines the posterior side of the buccal cavity better. In stage IV, the radular sac started as an invagination in the buccal floor and formed a small sac in stage V.
The larval stomach dilates more and the space between the body wall and the outher wall of the stomach is remarkably reduced. The cells of the stomach wall have increased and becomes stretched. At the right dorso-lateral side, the right gastric streak becomes conspicuous. The cells in this gastric streak will develop into portions oft the adult's gizzard. At the moment, the gizzard streak is best described as a small indented line accross the stomach wall.
On the left side of the primitive stomach, a small oval area appears on the wall. This area will develop into the sorting area of the gizzard of the snail in a later stage. The intestines at the end of the primitive stomach, bend sharply vantral, but still end blindly in the Marisa embryo. In Pila globosa, the intestine opening is situated mid-ventral.
The rudiments of the cerebral, pedal, intestinal and pleural ganglia detach from the ectoderm and form cell masses below the embryo's skin. The statocysts further invaginate from the ectoderm, a process that already started in stage V, and that will finally result in the formation of vesicles in future stages.
The pericard (heart sac) elonguates in antero-posterior direction (stretched in head-tail position) and becomes quite prominent in this stage. The formation of the auricle and ventricle inside the pericardial sac progresses and two distinct chambers are created. The auricle and ventricle only communicate through a small, developing valve between them.
Constriction between the pericard an dthe kidney takes place, and the posterior part of the kidney enlarges. The ureter increases in length and the U-shape becomes more prominent, with a contriction in the middle of the tube. The kidney itself moves to the left and finally lies at the medial line of the embryo.
According to Ranjah (1942), the first contraction in the heart region start in this stage, whereas Demian (1973) describes the first contractions in stage VII. The author of this website aggrees with Demian's description. It must be mentioned that the contractions described by Ranjah are not situated particularly in the heart itself, but in the integument between the foot and the anus. Also, of course Ranjah studied the embryology of the species Pila globosa, whereas Demian concentrated on Marisa cornuarietis, so there could be a difference with regards to early heart activity between these two species.


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