Figure 3

Alkaline phosphatase (AP) activity in the early shell gland of L. stagnalis . Endogenous alkaline phosphatase activity (dark blue precipitate in all panels) highlights the development of distinct cell populations and structures within the shell gland, and allows for the identification of distinct stages of larval shell development. A-A”(42 hpfc). The first evidence of AP activity in the shell gland occurs at 42 hpfc (arrows). B-B” (47 hpfc). Invagination of the shell gland begins at 47 hpfc. This is visible in AP⁺ cells which can be seen just below the outermost level of the dorsal ectoderm (arrow in B). C-C” (50 hpfc). Non-invaginated AP⁺ cells at the margin of the shell gland (white arrows in C) expand in an anterior direction (curved white arrows in C’). Invaginated AP⁺ cells (black arrows in C and C’) intensify their AP activity. The anlage of the protonephridia (pn) and apical plate also become AP⁺ at this stage. D-D” (52 hpfc). The non-invaginated AP⁺ cells at the margin of the shell gland continue to migrate in an anterior direction (curved white arrows in D’). All embryos are oriented with the shell field to the top. An asterisk marks the position of the blastopore. All scale bars are 20 μm. Numbers in the lower left of each panel row indicate the age in hours post first cleavage (hpfc).