f. foot; m. mouth; ce.v. cephalic vesicle; st. stomach.
With the growth of the shell and mantle the foot and the head become sharply separated from the visceral sack ([fig. 106]). The œsophagus (m) becomes elongated. The eyes and auditory sacks become formed.
With further growth the asymmetry of the embryo becomes more marked. The intestine takes a transverse direction to the right side of the body, and the anus opens on the right side and close to the foot in the mantle cavity which is formed by an epiblastic invagination in this region. The cavity of the stomach ([fig. 106], st) increases enormously and passes to the left side of the body, pushing the food-yolk at the same time to the right side, and the point where it communicates with the intestine becomes carried towards the posterior dorsal end of the visceral sack. The walls of the stomach gradually extend so as to narrow the opening to the yolk. The part of it adjoining the œsophagus becomes the true stomach, the remainder the liver; its interior is filled with coagulable fluid.
Paludina. Paludina—Lankester (No. [263]) and Bütschli (No. [244])—is a viviparous form characterised by the small amount of food-yolk. The hypoblast and epiblast cells are distinguished very early, but soon become of nearly the same size.
In the later stages of segmentation the epiblast cells differ from the hypoblast cells in the absence of pigment. The segmentation cavity, if developed, is small. A perfectly regular gastrula is formed ([fig. 107] A and B), which is preceded by the embryo assuming a flattened form. The blastopore is at first wide, but gradually narrows, and finally assumes a slightly excentric position. It becomes not the mouth, but the anus.
When the blastopore has become fairly narrow, mesoblast cells (B, me.) appear around it, between the epiblast and hypoblast. Whether they are bilaterally arranged or no is not clear; and though coloured like the hypoblast, their actual development from this layer has not been followed.
Fig. 107. Four stages in the development of Paludina vivipara.
(Copied from Bütschli.)
ep. epiblast; hy. hypoblast; me. mesoblast; bl. blastopore; an. anus; st. stomodæum; sh. shell-gland; V. velum; x. primitive excretory organ.