2)intra-embryonic blood vessels by the 18-20th days, endothelial tube network appears in intraembryonic mesenchyma to form intraembryonic endothelial tube network by the end of 3rd week, intraembryonic and extraembryonic endothelial tube networks connect to each other to form a diffuse endothelial tube network c-endothelial tube networks fuse or disappear to form primitive cardiovascular system2) intra-embryonic blood vessels ---by the 18-20th days, endothelial tube network appears in intraembryonic mesenchyma to form intraembryonic endothelial tube network ---by the end of 3 rd week, intraembryonic and extraembryonic endothelial tube networks connect to each other to form a diffuse endothelial tube network ---endothelial tube networks fuse or disappear to form primitive cardiovascular system