Cells Are Added to the Archenteron during and Following Secondary Invagination in the Sea UrchinLytechinus variegatus

In the present investigation, nuclei of endodermal cells, primary and secondary mesenchyme cells (PMCs and SMCs), and small micromere descendants (SMDs) of the sea urchinLytechinus variegatuswere counted and mapped at five developmental stages, ranging from primary invagination to pluteus larva. The archenteron and its derivatives were measured three dimensionally with STERECON analytical software. For the first time SMC production is included in the kinetic analysis of archenteron formation. While the archenteron lumen doubled in length during secondary invagination, the number of archenteron cells increased byat least38% (over 50% when SMCs that emigrated from the tip of the archenteron were included). The volume of the archenteron epithelial wall plus the volume of 17 new SMCs increased by 40% over the equivalent volumes at the end of primary invagination. Because secondary invagination involves the addition of archenteron cells and an increase in volume of the archenteron epithelium, we conclude that secondary invagination is not accomplished simply by the rearrangement and reshaping of the primary archenteron cells. Both archenteron cell number and wall volume continued to increase at the same rates from the end of secondary invagination until the 27-h prism stage, although the lumen lengthened more slowly. SMCs were also produced at a constant rate from primary invagination until the prism stage. Because the production of both endodermal and mesodermal cells continues until the late prism stage, we conclude that gastrulation (defined as the establishment of the germ layers) also extends into the late prism stage.