294 
ME. W. K. PAEKEE ON THE STETJCTUEE AND 
In this stage a section of the head (affected as it is hy the “ mesocephalic flexure”) 
which passes through the first cerebral vesicle and its upgrowth takes the pterygo- 
palatine rods almost from end to end, whilst the trabeculee are cut directly through 
(Plate XXIX. fig. 5, tr.p.pg.). Thus, as compared with the trabecular apices, the pterygo- 
palatines descend a little before they send upwards the apical hook. 
Third Arch , or First Postoral. — This rod, like its immediate successor, is stout, 
sigmoid, and strongly inhooked above; it does not at present meet its fellow at the 
mid line. This is the primordial mandible, mn. ; but it remains as the lower jaw for a 
very short time, and is not segmented into an upper and lower piece. There is a stage 
in all the oviparous Vertebrata in which this rod is free from segmentation ; but, above 
the Lamprey, a pier and free arch are formed by subdivision of the bar. The tissue over 
it is thick, and in this overlying part the persistent mandible is formed (see Plate 
XXVIII. figs. 1, 4, & 6, and Plate XXIX. figs. 5 & 6, ml.). 
The morphological changes that take place in the hooked and inbent apex are of the 
greatest interest ; for now we arrive at the point where not only the hyoid arch is arrested 
and modified in relation to the outworks of the organ of hearing, but the mandible of 
the embryo is also suddenly given up to these secondary correlations. Considered in 
relation to their new function, the parts of the mandible of the mammal might, like 
those of the upper part of the hyoid arch, be included in the stapedian terminology*. 
The Meckelian rod itself is shown in the vertical section near its extremity (Plate 
XXVIII. fig. 6, ml.), and in the palatal view (fig. 4, ml.) near its apex ; near its apex it 
is seen on the outside in the lateral view of the sliced head (Plate XXIX. fig. 6, ml.). But 
horizontal sections of the head are necessary to show the relation of the apex of this bar 
to the first postoral cleft, the rudimentary ear-drum cavity (see Plate XXIX. figs. 7, 8, 
9, ml.). These sections show that this very expanded cleft is being divided into two 
spaces, one of which (the inner) becomes the tympanic cavity, and the other the “ meatus 
auditorius externus.” The septum or diaphragm is formed by the lining skin of the cleft 
growing outwards from the side of the ear-sac, and inwards from the outer face ; this 
latter growth is the most intense, being pushed in by the ingrowth of the apex of the 
embryonic mandible, which, growing inwards and backwards, carries the lining skin of 
the cleft before it; thus the “ membrana tympani” is formed. Looking at these 
figures, we see at once that the “manubrium mallei” is the hooked apex of the primor- 
dial mandibular arch, and that therefore it must correspond with the large bifaceted 
backwardly placed head of the Bird’s quadrate bone”f. 
The shoulder or tuberculum of this rib-like bar becomes the thick head of the 
* See “ On the Representatives of the Malleus and the Incus of the Mammalia in the other Yertebrata,” hy 
Professor Huxley, Proc. Zool. Soc., May 27, 1869, pp. 391-407. 
f I was under the impression that the “internal angular process” of the Bird’s mandible (“Fowl’s 
Skull,” Plate ixxxi. fig. 13, i.a.p .) was the homologue of the manubrium mallei of the mammal ; it is not ; both 
it and the posterior process ( p.a.p .) are outgrowths formed lower down, and correspond in nature to the 
“ opercular knob” of the next or hyoid arch. 
