'allantois-lacking' originates from combining the Neo-Latin noun 'allantois' (from Greek 'allantoeis' meaning 'sausage') and the English word 'lacking' (from Middle English 'lack' ultimately related to Old Norse/Old English roots meaning 'absence').

'allantois' entered English via Neo-Latin from Greek 'allantoeis' (originally describing a 'sausage-shaped' structure); 'lacking' evolved from Middle English 'lack' and adopted its modern form in Early Modern English, and the compound 'allantois-lacking' was formed in modern scientific usage to describe organisms without an allantois.

Initially, 'allantois' referred specifically to the sausage-shaped embryonic sac; over time the compound 'allantois-lacking' came to be used in technical descriptions to mean 'without that sac' in embryos.