Ossicular poions found within the mesotympanum embryologically originate from the following:

The ossicular poions found in the attic are formed from the first branchial arch. This includes the head of the malleus and the body and sho process of the incus. The ossicular poions that are found within the mesotympanum originate from the second branchial arch. This includes the long process of the malleus, the long process of the incus, and the stapes superstructure. The stapes footplate originates from the otic capsule (the primordial otocyst), rather than from a branchial arch. The ossicles are full-sized cailage models by 15 weeks of gestation, and endochondral ossification is complete by 25 weeks. The middle ear is adult sized at bih. Ref: Oghalai J.S., Brownell W.E. (2012). Chapter 44. Anatomy & Physiology of the Ear. In A.K. Lalwani (Ed), CURRENT Diagnosis & Treatment in Otolaryngology--Head & Neck Surgery, 3e.