The following section from the Curtis Treatise on the Law of Patents is interesting in my estimation.
Section 170 The statutes however which authorize the assignment of an invention before the patent has been obtained appear to embrace only the cases of perfected or completed inventions. There can properly speaking be no assignment of an inchoate or incomplete invention although a contract to convey a future invention may be valid and may be enforced by a bill for a specific performance. But the legal title to an invention can pass to another only by a conveyance which operates upon the thing invented after it has become capable of being made the subject of an application for a patent This is apparent from the provisions of the statute which require the specification and the application to be made in the name of the inventor. A contract to convey a future invention or an improvement to be made upon a past invention cannot alone authorize a patent to be taken by the party in whose favor such contract was intended to operate.
George T. Curtis, A Treatise on the Law of Patents for Useful Inventions (1873).