Fig. 1. Fertilization in chaetognaths. (A) Spadella cephaloptera: adult specimen with the two testes (T) and the two ovaries (O) in the posterior and anterior part of the body cavity, respectively. The two rows of maturing oocytes are colored pink and the two seminal receptacles (SR) and sperm ducts (SD) are colored blue. (B) Sagitta inflata: oocyte (pink) just before fertilization. The germinal vesicle (GV) has migrated to the animal pole opposite the fertilization apparatus (yellow). (C) Sagitta inflata: a portion of the sperm duct (SD) filled with sperm (blue). Bundles of sperm (arrows) traverse the sperm duct wall in the direction of oocytes. (D) Sagitta inflata: first polar body (PB) emission just before fertilization. (E) Sagitta inflata: a bundle of sperm (blue) reaches the fertilization apparatus. The AFC2 cell is colored yellow. The nucleated portion of AFC2 cell contacts the oocyte (pink) while the rest of the cell spirals (arrow) and contacts the sperm bundle (blue). (F) Sagitta setosa: bundles of sperm (Sp, blue) traverse the sperm duct (SD) wall in the direction of the microtubule (Mt)-filled cellular extension of the AFC2 cell rich in vesicles (arrow). (G,H) Sagitta setosa: close-up view of the cellular extension of the AFC2 cell showing the microtubule bundles before fertilization (G) and the abundance of vesicles just after fertilization (H). (I) Sagitta bipunctata: sequence of sperm penetration in the region of the AFC2 cell (only the nuclear portion is colored yellow). The AFC2 cell resorbs. (J) Sagitta bipunctata: same as I but at a higher magnification. The entering sperm (Sp) can be observed.