Fig. 105. Longitudinal section through the embryo of Nassa mutabilis.
(After Bobretzky.)
A. Stage when the mesoblast is commencing to be formed.
B. Stage when the yolk is half enclosed. The hypoblast is seen at the lips of the blastopore.
C. Stage when the blastopore (bp) is nearly obliterated.
D. The blastopore is closed.
ep. epiblast; me. mesoblast; hy. hypoblast; bp. blastopore; in. intestine; st. stomach; f. foot; sg. shell-gland; m. mouth.
Two definite organs have already become established. One of these is a pit lined by thickened epiblast on the posterior and dorsal side (sg). This is the shell-gland. The other is the foot (f) which arises as a ventral prominence of thickened epiblast immediately behind the blastopore. The hypoblast forms a ring of columnar cells round the blastopore. On the posterior side its cells have bent over so as to form a narrow tube (in), the rudiment of the intestine.
In the next stage ([fig. 105] D) the blastopore completely closes, but its position is marked by a shallow pit (m) where the stomodæum is eventually formed. The foot (f) is more prominent, and on its hinder border is formed the operculum. The shell-gland (not shewn in the figure) has flattened out, and its thickened borders commence to extend especially over the dorsal side of the embryo. A delicate shell has become formed. In front of and dorsal to the mouth, a ciliated ring-shaped ridge of cells, which is however incomplete dorsally, gives rise to the velum. On each side of the foot there appears a protuberance of epiblast cells, which forms a provisional renal organ. The hypoblast now forms a complete layer ventrally, bounding a cavity which may be conveniently spoken of as the stomach (st), which is open to the yolk above. Posteriorly however a completely closed intestine is present, which ends blindly behind (in).
The shell and with it the mantle grow rapidly, and the primitive symmetry is early interfered with by the shell extending much more towards the left than the right. The anus soon becomes formed and places the intestine in communication with the exterior.
Fig. 106. Longitudinal section through an advanced embryo of Nassa mutabilis. (After Bobretzky.)