_ch._, notochord; _iv._, parachordals; _au._, auditory capsule; _py._, pituitary body; _tr._, trabecula; _ctr._, trabecular cornu; _pn._, pre-nasal cartilage; _ppg._, palato-pterygoid tract; _mn._, mandibular arch; _th.h._, first branchial arch; _VII.-XII._, cranial nerves.] [Illustration: FIG. 49.--HEAD OF EMBRYO DOG-FISH (from PARKER), BASAL VIEW OF CRANIUM FROM ABOVE. _ol._, olfactory sacs; _au._, auditory capsule; _py._, pituitary body; _pa.ch._, parachordal cartilage; _tr._, trabecula; _inf._, infundibulum; _pt.s._, pituitary space; _e._, eye.] 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 posteriorly 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 (_pt.s._). In {122}front of this space they remain in contact, and generally unite. They extend forward into the nasal region (_pn._). 3. The cartilaginous capsules of the sense organs. Of these the auditory (_au._) and the 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 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.