Quarterly Journal of Microscopical Science, Vol s2-74, 591-646, Copyright © 1931 by Company of Biologists

Memoirs: The Development of the Skull of Scyllium (Scyliorhinus) canicula L

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

1. The development of the skull of Scyllium canicula has been studied from the first appearance of cartilage, through thirteen stages, up to the point at which the main features of the adult skull have been acquired.

2. The parachordals are the first elements to chondrify, and evidence is presented confirming Goodrich's observations concerning the visible traces of metameric segmentation of the metotic region of the paraohordal.

3. The auditory capsule chondrifies from the first in continuity with the parachordal, to which it is attached by the anterior basicapsular commissure.

4. The polar cartilages have not been found separate, but they appear as nodules of cartilage attached to the under surface of the anterior ends of the parachordals.

5. The orbital cartilage becomes attached to the parachordal by means of the pila antotica, and to the trabecula at the base of the lamina orbitonasalis by means of the preoptic root.

6. The hind wall of the pituitary fossa is formed in a complex manner, from a postpituitary commissure between the polar cartilages, and a pair of inwardly directed processes from the foremost ends of the parachordals forming the dorsum sellae. There is also a precarotid commissure, enclosing the carotid arteries in a foramen between itself and the postpituitary commissure.

7. The basicranial fenestra has been demonstrated.

8. Arguments are given for rejecting Allis's view that the so-called basicranial fenestrae throughout the craniates are not homologous.

9. Attention is called to the vacuity in the median wall of the auditory capsule through which the posterior canal bulges, and to the fact that this vacuity is not to be confused with the foramen endolymphaticum.

10. The relations of the glossopharyngeal nerve are described, and it is shown that its apparent passage through the cavity of the auditory capsule is to be ascribed to the fact that the lamina hypotica of the parachordal acts as a false floor to the auditory capsule, the true floor of which is in this region unchondrified.

11. The problem of the relations of the jaws to the brain-case is reviewed in the light of recent investigations, and a reasoned classification is attempted.

12. It is noticed that chondrification is delayed in embryonic material collected from Naples, as compared with material of similar size and degree of development obtained from Plymouth.