Early development of the facial nerve in the chick embryo with special reference to the development of the chorda tympani.

The development of the facial nerve from Hamburger and Hamilton stage 17 to stage 28 is described in chick embryos by means of a new immunochemical nerve staining method that uses an antineurofilament protein (NFP) antibody. A postspiracular branch and an unknown transient posterior branch beneath the ostocyst were observed at stage 17. At stage 19, the primordia of the r. palatinus were observed. A prespiracular branch appeared at stage 21, and with the postspiracular nerve, it made a loop encircling the spiracle (spiracular loop). The first primordium of the ramus (r) hyoideus and transient rami (rr) dorsales appeared around stage 23. At stage 25, the chorda tympani was first observed to arise from the ventral end of the spiracular loop. At stage 26, a communicating branch, connexus cum nervo glossopharyngeo, was found along with the vena (v) capitis lateralis. The rr. dorsales seemed to represent the r. supratemporalis in lower animals. The communicating branches around the v. capitis lateralis seemed to correspond to the cutaneous nerve communications between the branchial nerves frequently encountered in Amphibia. It was found that the chorda tympani becomes a prespiracular nerve for the most part in the chick by the reduction of the postspiracular component of the spiracular loop. Thus, the nerve differs markedly from that in other animals, which is postspiracular. This difference explains the different passage of this nerve in the chick as compared with other amniotes.