Figure 8.

A schematic representation of the major events during early development of the shell gland and shell field in L. stagnalis. Uppermost row are all dorsal views except for ~87 hpfc which is a lateral view. Lowermost row are the corresponding transverse sections through the developing shell gland and shell field. By approximately 32 hours post first cleavage (hpfc) endodermal cells at the tip of the archenteron, the so called“small celled endoderm” for their lack of large vacuoles (indicated by black ovals and circles), have made contact with the overlying cells of the dorsal ectoderm. These are the only endodermal cells to make contact with ectoderm. By 42 hpfc cells that are in contact with the endoderm have thickened and some cells display alkaline phosphatase (AP) activity. The strongest AP activity is apparently in extra-cellular material. A bilateral invagination of the shell gland has also commenced at this age. By 47 hpfc the bilateral invagination has deepened and non-invaginated, posterior shell gland cells at the periphery of the shell gland also display AP activity. At 52 hpfc the bilateral invaginations have fused, and the lumen of the shell gland displays intense AP activity. By 62 hpfc the lumen of the shell gland appears to be sealed with intense AP activity. The first peroxidase (PO) activity is visible at this time and is evident in cells and in the secreted periostracum material. By 87 hpfc the form of the juvenile snail has been established and the non-overlapping zones of AP and PO activity are maintained.