原始生殖细胞来自极细胞，分两团移至胚胎腹部侧背面，被中胚层细胞包裹，形成生殖脊。
Primordial germ cells came from pole cells and moved in two masses to the laterodorsal abdomen of embryo , and then were enveloped by mesoderm to form germinal ridge .
侧中胚层