(2) A pair of bars forming the floor for the fore-brain, known as the trabeculæ (tr). These bars are continued forward from the parachordals. They meet behind and embrace the front end of the notochord; and after separating for some distance bend in again in such a way as to enclose a space—the pituitary space. In front of this space they remain in contact and generally unite. They extend forwards into the nasal region (pn).
(3) The cartilaginous capsules of the sense organs. Of these the auditory (au) and olfactory capsules (ol) unite more or less intimately with the cranial walls; while the optic capsules, forming the usually cartilaginous sclerotics, remain distinct.
The parachordals and notochord. The first of these sets of elements, viz. the parachordals and notochord, forming together the basilar plate, is always an unsegmented continuation of the axial tissue of the vertebral column. It forms the floor for that section of the brain which belongs to the primitive postoral part of the head (vide p. 314), and its extension is roughly that of the basioccipital of the adult skull. Its mode of development is almost identical with that of the vertebral column, except that the notochord, even in many forms where it persists in the vertebral column, disappears in the basilar plate; though in a certain number of cases remnants of it are found in the adult state.
Fig. 324. Longitudinal section through the brain of a young Pristiurus embryo.
cer. commencement of the cerebral hemisphere; pn. pineal gland; In. infundibulum; pt. ingrowth from mouth to form the pituitary body; mb. mid-brain; cb. cerebellum; ch. notochord; al. alimentary tract; Iaa. artery of mandibular arch.
It will be convenient to say a few words here with reference to the notochord in the head. It always extends along the floor of the mid- and hind-brains, but ends immediately behind the infundibulum. The limits of its anterior extension are clearly shewn in [fig. 43]. The front end of the notochord often becomes more or less ventrally flexed in correspondence with the cranial flexure; its anterior end being in some instances (Elasmobranchii) almost bent backwards ([fig. 324]).
Kölliker has shewn that in the Rabbit[203], and I believe that a more or less similar phenomenon may also be observed in Birds, the anterior end of the notochord is united to the hypoblast of the throat in immediate contiguity with the opening of the pituitary body; but it is not clear whether this is to be looked upon as the remnant of a primitive attachment of the notochord to the hypoblast, or as a secondary attachment.
Before the parachordals are formed the anterior end of the notochord has usually undergone a partial atrophy; and its front end often becomes somewhat dorsally flexed. Within the basilar plate it often exhibits two or more dilatations, which have been regarded by Parker and Kölliker as indicative of a segmentation of this plate; but they hardly appear to me to be capable of this interpretation.
In Elasmobranchs where, as shewn above, a very primitive type of development of the vertebral column is retained, we find that the basilar plate is at first formed of (1) the notochord invested by its cartilaginous sheath, and (2) of lateral masses of cartilage, the parachordals, homologous with the arch tissue of the vertebral column. This development probably indicates that the basilar plate contains in itself the same elements as those from which the neural arches and the centra of the vertebral column are formed; but that it never passes beyond the unsegmented stage at first characteristic of the vertebral column. The hinder end of each parachordal forms a condyle articulating with the first vertebra; so that in the cartilaginous skull there are always two occipital condyles. The basilar plate always grows up behind ([fig. 326], so), and gives rise to a complete cartilaginous ring enveloping the medulla oblongata, in the same manner that the neural arches envelope the spinal cord. This ring forms an occipital cartilaginous ring; in front of it the basilar plate becomes laterally continuous with the periotic cartilaginous capsules, and the occipital ring above usually spreads forward to form a roof for the part of the brain between these capsules. In the higher Vertebrates the periotic cartilages may be developed continuously with the basilar plate ([fig. 325]).