Early shell development in L. stagnalis illustrated by Scanning Electron Microscopy (SEM). A-C’ Gastrulation and formation of the archenteron. The site of the future shell gland is marked by white arrowheads. The blastopore is marked by an asterisk. D-E’ The first outward signs of shell gland invagination are two shallow depressions at 42 hpfc (arrowheads in D’ and E’). F-I’ Insoluble material secreted by the shell gland is visible from 52 hpfc onwards. The first asymmetry of the shell is evident at 62 hpfc (highlighted by two dashed ovals in H’). All scale bars are 10 μm. Numbers in the lower left of each panel indicate the age in hours post first cleavage (hpfc). Panel G is reflected about the vertical axis for consistency of presentation.
Hohagen and Jackson BMC Developmental Biology 2013 13:27 doi:10.1186/1471-213X-13-27