The parachordals and notochord form together the basilar plate, which forms the floor for that section of the brain belonging to the primitive postoral part of the head, and its extent corresponds roughly to that of the basioccipital of the adult skull.
The trabeculæ, so far as their mere anatomical relations are concerned, play the same part in forming the floor for the front cerebral vesicle as do the parachordals for the mid- and hind-brain. They differ, however, from the parachordals in one important feature, viz. that except at their hinder end they do not embrace the notochord. The notochord always terminates at the infundibulum, and the trabeculæ always enclose a pituitary space, in which lies the infundibulum (inf.) and the pituitary body (py.).
In the majority of the lower forms the trabeculæ arise quite independently of the parachordals, though the two sets of elements soon unite.
The trabeculæ are usually somewhat lyre-shaped, meeting in front and behind, and leaving a large pituitary space between their middle parts. Into this space the whole base of the fore-brain primitively projects, but the space itself gradually becomes narrowed until it usually contains only the pituitary body.
The trabecular floor of the brain does not long remain simple. Its sides grow vertically upwards, forming a lateral wall for the brain, in which in the higher types, two regions may be distinguished, viz. an alisphenoidal region behind, growing out from what is known as the basisphenoidal region of the primitive trabeculæ, and an orbito-sphenoidal region in front, growing out from the presphenoidal region of the trabeculæ. These plates form at first a continuous lateral wall of the cranium. The cartilaginous walls which grow up from the trabecular floor of the cranium generally extend upwards so as to form a roof, though almost always an imperfect roof, for the cranial cavity.
The basi-cranial cartilaginous skeleton reduces itself always into trabeculæ and parachordals with olfactory and auditory cartilaginous capsules.
In addition, a branchial skeleton exists, which consists of a series of bars known as the branchial bars, so situated as to afford support to the successive branchial pouches. An anterior arch known as the mandibular arch (Fig. [50], Mn.), placed in front of the hyo-mandibular cleft, and a second arch, known as the hyoid arch (Hy.), placed in front of the hyo-branchial cleft, are developed in all types; the succeeding arches are known as the true branchial arches (Br.), and are only fully developed in the Ichthyopsida. In all cases of jaw-bearing (gnathostomatous) vertebrates the first arch has become a supporting skeleton for the mouth (Fig. [51]), and in the higher vertebrates in combination with the second or hyoid arch takes part in the formation of the ear-bones.
Fig. 50.—Head of Embryo Dog-fish, eleven lines long. (From Parker.)
Tr., trabecula; Mn., mandibular cartilage; Hy., hyoid arch; Br1., first branchial arch; Na., olfactory sac; E., eye; Au., auditory capsule; Hm., hemisphere; C1, C2, C3, cerebral vesicles.