Figure 8 in Position and development of oocytes in velvet worms shed light on the evolution of the ovary in Onychophora and Arthropoda

Figure 8. Origin and fate of primordial germ cells in Epiperipatus biolleyi (A, B) and Opisthopatus roseus (C, D). Transmission electron micrographs. A, primordial germ cells (pgc) that occur as an unpaired cluster of cells at the posterior end of an early segmenting embryo. Inset shows the position of the cluster (arrow) in a slightly older elongating embryo. B, horizontal section of a coelomic cavity with primordial germ cells associated with the visceral coelomic lining (cl) in an embryo with a full number of somites developed. Anterior is up, mid-line is left. C, horizontal section of a coelomic lining with associated primordial germ cells in an embryo with the full number of somites. D, detail of a primordial germ cell, which is covered by slender processes of coelomic lining cells towards the coelomic cavity (co). Arrowhead indicates an apical junction. Abbreviations: bc, primary body cavity; cl, coelomic lining cells; co, coelomic cavity; en, endoderm; mt, mitochondria; nu, nucleus; pgc, primordial germ cells.