Figure 4

Alkaline phosphatase (AP) and peroxidase (PO) activity in the mature shell gland of L. stagnalis . Endogenous AP activity intensifies as the shell gland matures, and PO activity also becomes detectable. A-A”’ (57 hpfc). The mature shell gland at 57 hpfc is characterised as a closed lumen with intense AP activity (white dot in A). AP activity has also increased in non-invaginated cells at the margin of the shell gland (white arrows in A), and in the anlage of the protonephridia (pn). Weak PO activity is also evident at this stage (black arrows in A’ and A”’), and secreted insoluble material can also be seen in preparations of this age (arrowhead in A’). Non-invaginated AP⁺ cells at the margin of the shell gland finish their anterior expansion, meeting at the midline (curved white arrows in A”). B-B”’ (62 hpfc). In 62 hpfc larvae, the shell gland is maintained as a closed lumen (white dot in ) while PO activity in non-invaginated cells intensifies (black arrows in B’). C-C”’(67 hpfc). Between 62 and 67 hpfc evagination of the shell gland has commenced (transparent white arrows in C) and PO activity in non-invaginated cells of the shell gland margin has increased (black arrow in C’). The asymmetry of the shell gland is made clearly visible by populations of AP⁺ and PO⁺ cells (dashed ovals in C” and C”’ respectively). All embryos are oriented with the shell gland 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).