▁retre at ▁of ▁the ▁ice ▁sheets . ▁ ▁Re production ▁M ating ▁occurs ▁on ▁land , ▁the ▁male ▁cl asp ing ▁the ▁female ▁at ▁the ▁fore ▁legs , ▁and ▁the ▁fert il ization ▁is ▁internal . ▁S . ▁at ra ▁is ▁a ▁viv ip ar ous ▁am ph ib ian , ▁giving ▁birth ▁to ▁two ▁live ▁young , ▁or ▁rarely ▁three ▁or ▁four . ▁They ▁may ▁measure ▁as ▁long ▁as ▁ ▁at ▁birth , ▁with ▁the ▁female ▁meas uring ▁only ▁. ▁The ▁u ter ine ▁eggs ▁are ▁large ▁and ▁numerous , ▁but , ▁as ▁a ▁rule , ▁only ▁one ▁fully ▁develop s ▁in ▁each ▁u ter us , ▁the ▁emb ry o ▁being ▁n our ished ▁on ▁the ▁y olk ▁of ▁the ▁other ▁eggs , ▁which ▁more ▁or ▁less ▁dissol ve ▁to ▁form ▁a ▁large ▁mass ▁of ▁nut ri ent ▁matter . ▁The ▁emb ry o ▁passes ▁through ▁three ▁stages : ▁still ▁en closed ▁within ▁the ▁egg ▁and ▁living ▁on ▁its ▁own ▁y olk ▁free , ▁within ▁the ▁vit ell ine ▁mass , ▁which ▁is ▁directly ▁sw allowed ▁by ▁the ▁mouth ▁with ▁no ▁more ▁vit ell ine ▁mass , ▁the ▁emb ry o ▁is ▁possessed ▁of ▁long ▁external ▁g ills , ▁which ▁serve ▁as ▁an ▁exchange ▁of ▁nut rit ive ▁fluid ▁through ▁the ▁mat ernal ▁u ter us , ▁these ▁g ills ▁function ing ▁in ▁the ▁same ▁way ▁as ▁the ▁ch or ionic ▁vill i ▁of ▁the ▁m amm al ian ▁egg ▁ ▁Gener ally , ▁at ▁alt itudes ▁of ▁ 6 5