Usually, rudiments.

the elements or first principles of a subject

the rudiments of grammar.

a mere beginning, first slight appearance, or undeveloped or imperfect form of something

the rudiments of a plan.

Biology . an organ or part incompletely developed in size or structure, as one in an embryonic stage, one arrested in growth, or one with no functional activity, as a vestige.

noun

base , cornerstone , foundation , fundament , fundamental , root , basic , essential