Fig. 119. Three views of an embryo of Pisidium immediately after the closure of the blastopore. (After Lankester.)
A. View from the surface.
B. Optical section through the median plane.
C. Optical section through a plane a little below the surface.
ep. epiblast; me. mesoblast; hy. hypoblast; p. cells apparently budding from the hypoblast to form mesoblastic elements.
The embryo now rapidly grows in size. The blastopore becomes closed and the archenteric sack forms a small mass attached at one point to the walls of the embryonic vesicle ([fig. 119], hy). In the space between the walls of the archenteron and those of the embryonic vesicle stellate mesoblast cells make their appearance, derived in the main from the epiblast, though probably in part also from the hypoblastic vesicle (vide [fig. 119] C, p). The cavity between the hypoblast and epiblast, which contains these cells, is the body cavity. [Fig. 119] represents three views of the embryo at this stage. A is a surface view shewing the epiblast; B is an optical section through the median plane shewing the hypoblast and some of the mesoblast cells; and C is an optical section shewing the mesoblast cells. A prominence on one side of the embryo now develops which forms the commencement of the foot, and the archenteric sack grows out at its free extremity into two lobes, but remains attached to the epiblast by an imperforate pedicle. The next organ to appear is the stomodæum. It arises as a ciliated epiblastic ingrowth which meets the free end of the archenteric sack, fuses with it, and shortly afterwards opens into it ([fig. 118], ph). Between the mouth and the attachment of the enteric pedicle is placed the foot (f), which becomes ciliated. On the dorsal side of the enteric pedicle there appears a saddle-shaped patch of epiblast cells bounding the sides of a groove (shs). This is the rudiment of the shell-gland.
The enteric pedicle, or intestine as it may now be called, soon acquires a lumen, though still imperforate at its termination where the anus is eventually formed. Ventral to the intestine is placed a mass of cells—the rudiment of the organ of Bojanus. It is stated to be developed as an ingrowth of the epiblast.
In a slightly later stage the shell-gland rapidly increases in size and flattens out, and on the two sides of it there appear the rudiments of the two valves, which are at first quite distinct, and separated by a considerable interval ([fig. 120]). Before the appearance of the valves of the shell, the mantle folds have already grown out from the sides of the body.
Fig. 120. Diagrammatic view of advanced larva of Pisidium. (Copied from Lankester.)
m. mouth; a. anus; B. organ of Bojanus; mn. mantle; f. foot.