'non-allantoid' is a compound formed by the prefix 'non-' (from Latin 'non', meaning 'not') plus 'allantoid' (from New Latin 'allantoides', ultimately from Greek roots meaning 'sausage-shaped').

'allantois' entered scientific New Latin from Greek (via terms describing a sausage-shaped structure), producing New Latin 'allantoides'/'allantoid', which passed into modern English as 'allantoid'; the modern compound 'non-allantoid' was formed by adding the negative prefix 'non-' to 'allantoid'.

Originally referring to a sausage-shaped structure (Greek sense), the term 'allantoid' came to mean 'resembling or relating to the allantois'; prefixed with 'non-' it now denotes the negation 'not resembling or relating to the allantois'.