150 
ME. W. K. PAEKEE ON THE STEHCTUEE AND 
were so totally unrelated. The parallel faces of the ends of the trabeculae in fig. 5 
cannot have coalesced wholly, but only behind — the free anterior rounded angle growing 
into the leaf-like trabecular cornua (fig. 9, tr.c.). 
That I have not overrated the rapidity nor the kind and degree of the changes which 
have taken place in three or four days will be seen if fig. 2 be compared with fig. 9, as 
to the apex of both the first and second arch (trabecular and mandibular). 
In fig. 2 we see that these tops are apiculated, inturned, free, and a long distance from 
the pituitary body (py.) ; in fig. 9 they have become wholly fused at the top, have nearly 
gained the mid line behind the . pituitary body, and although distinct above from the 
investing mass (i.v.), embracing it, and forming a retral lobe, yet, seen from below (fig. 10, 
tr., m.n., i.v.) we have clear proof that these facial bars have now grafted themselves upon 
the investing mass, skull and face being now no longer distinct. 
In fig. 9 the eyeball ( e ) and its surrounding stroma hide the pterygo-palatine bar; 
but in fig. 10 this is seen. 
The top of the second postoral, or hyo-mandibular ( h.m .), is now seen to be just 
inside the fore part of the pyriform auditory capsule (au.) ; it is close to it and is creep- 
ing outwards, ready to coalesce and be carried to the outside of the face by that part of 
the capsule which contains the ampulla of the horizontal semicircular canal. Behind 
the hyo-mandibular the section (fig. 9, br. 1) has displayed the top of the fourth arch, 
or first branchial ; it is the pharyngo-branchial portion ; and its counterpart is well known 
in Fishes, after attaching itself, quite normally, below the prootic region of the cranium. 
We can now see the structures on which the membranous cranium rests (figs. 8 and 9) — 
namely, the cephalic part of the notochord (n.c.), its investing mass (i.v.), and the auditory 
capsules (au.) ; the division between the cephalic and vertebral portions of the axial 
skeleton now more clearly displays the occipito-atlantal articulation, the ends of the 
investing mass forming the paired occipital condyles. The nature of the vertebrate 
skull, as, morphologically, highly compound, is well shown in these horizontal views. 
Even the floor of the pituitary space, the fundus of the “ sella turcica,” is craniofacial in 
its nature. In fig. 10 it is seen that cartilage is creeping beneath the pituitary body into 
the substance of that diverticulum of the membranous cranium which contains this part 
of the brain. Now this centripetally growing floor is derived, behind, from the “ investing 
mass,” laterally from the trabecuke, and in front from their secondary commissure. In 
this figure, which is solid, and seen from below, the cranio-facial base is exposed up to the 
front of the “ commissure;” but between this part and the severed upper lip with its carti- 
lages, the fore part of the palatal roof is shown with the first pair of visceral clefts, namely 
those which form the nasal passages. The meaning of these parts will be better shown 
in the perfect Tadpole, the next stage ; but here the swelling of the mucous membrane, 
mesiad of the opening, is seen to be due to the overlying trabecular horn which is inside 
the nasal passage ; the pterygo-palatine connective (pg.p.) bounds the passage behind ; 
and the quadrate condyle (q.), here cut away, lies on its outside. The connective which 
binds the metapterygoid part of the mandibular pier to the investing mass turns suddenly 
