Evolutionary aspects of primordial germ cell formation.

摘要: Animal embryos can be classified into three types depending on the time when adult body form is specified--after metamorphosis, progressively by addition of posterior segments, or as a single event early in development. Segregation germ cells correlates with specification form. When specified late development, e.g. after metamorphosis (molluscs, echinoderms, cirripedes, hemichordates, cephalochordates and ascidians), appear at site where gonads will develop. during development sequential segments (annelids, onychophorans most arthropods) are segregated either before close association growth zone. In nematodes, chaetognaths, collembolans, higher holometabolous insects vertebrates, typically correspondingly extraembryonic regions. Therefore, general conclusion, to locations and/or times that exclude them from process Germ plasm restricted which exclusion difficult because embryo small signal specifying pervasive. A possible role for thus additional protection processes