Quarterly Journal of Microscopical Science, Vol s2-74, 701-536, Copyright © 1931 by Company of Biologists

Memoirs: On the Nature of the Trabecula Cranii

¹ Fellow of Merton College, Jenkinson Lecturer in Embryology, and Demonstrator in Zoology in the University of Oxford

1. The existing evidence concerning the origin and nature of the trabecula cranii is reviewed, and it is shown that it constitutes a prima facie case for supporting Huxley's opinion that it represents a visceral structure.

2. The origin of the trabecula is studied in Scyllium canicula, Salmo fario, Rana temporaria, and Amblystoma tigrinum, and the results of this investigation support Huxley's opinion.

3. The grounds for adhering to Huxley's view are chiefly that: the trabecular rudiment is a mesenchymatous condensation in the maxillary process; there is no evidence of the trabecular rudiment being derived from the somites; the trabecular rudiment is closely associated with that of the pterygo-quadrate; if the trabecular rudiment is of sclerotomic origin, then the palatine process of the pterygo-quadrate and the mesenchyme of the ventral side of the front of the head must also be derived from the sclerotonies: an impossible conclusion.

4. The implications of the recognition of the trabecula as a premandibular arch are considered, and it is concluded that the mouth of Gnathostomata represents the original velar perforation of Amphioxus which has extended to the side and obliterated a pair of mandibular clefts or the dermal pouches corresponding to them.

5. Sewertzoff's view that the brain-case of Petromyzon is wholly chordal in composition is supported, and the homologues of the trabeculae are represented by a pair of premandibular visceral arches.