. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. GENETIC REGULATORY NETWORKS 369 Caenogastropoda 24-40 Architaenioglossa 44-48. Euthyneura 24 Valvatoidea 40 ancient Archaeogastropoda 63 Figure 1. Phylogenetic relations among different gastropod taxa based upon the number of cells in the embryo at the time of the mesen- toblast (3D) division (the number of cells is written under the boxed taxon name). The mesentoblast divides into the mesodermal stem cell (4d) and an endoderm precursor (4D). Caenogastropods. together with the Architaenioglossa, probably constitute a separat
. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. GENETIC REGULATORY NETWORKS 369 Caenogastropoda 24-40 Architaenioglossa 44-48. Euthyneura 24 Valvatoidea 40 ancient Archaeogastropoda 63 Figure 1. Phylogenetic relations among different gastropod taxa based upon the number of cells in the embryo at the time of the mesen- toblast (3D) division (the number of cells is written under the boxed taxon name). The mesentoblast divides into the mesodermal stem cell (4d) and an endoderm precursor (4D). Caenogastropods. together with the Architaenioglossa, probably constitute a separate group, as they form polar lobes during at least the first two cleavages. (After van den Biggel- aar and Haszprunar, 1996, ©Allen Press, used with permission) formation in nemertean and flatworm embryos show simi- larities and differences compared to molluscs and annelids and to each other. The nemertean embryo is not divided into dorsal, ventral, right, and left quadrants, but into two dorsolateral and two ventrolateral quadrants (9). Despite this alternative quadrant arrangement with respect to the first cleavage planes, bandlets of mesenchymal cells seem to be derived from the same endomesodermal cell (4d) as in annelids and molluscs (10). Like molluscs and annelids, flatworm embryos are also divided into dorsal, ventral, and two lateral quadrants; the specification of the dorsal quadrant, however, must be different. After the formation of the fourth quartet of micromeres, the micromeres 4a- 4d extend in the animal direction, in contrast to the mac- romeres 3A-3D in molluscan and annelid embryos. Fi- nally, it is the micromere of the ventral quadrant (4b) that maintains the contacts with the animal micromeres (van den Biggelaar, unpub. obs.). Micromere 4d of the opposite dorsal quadrant then develops the mesentoblast. These differences in mesentoblast formation between annelids, molluscs, and nemerteans on the one hand, and flatworms on the other hand, again demonstrate th
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