The term 'Rudimentary Stage' refers to the early stages of development in an EMBRYO. The term is often used to describe the earliest stages of development in a fetus, when the EMBRYO is still in its most basic form. The term can also be used to refer to the early stages of development in any organism, such as a plant or animal. The crossword solution for this term is 'EMBRYO, ' which is a six-letter word that describes the earliest stage of development in an organism.