294 



ME. W. K. PAEKEE ON THE STETJCTUEE AND 



In this stage a section of the head (affected as it is by the " mesocephalic flexure") 

 which passes through the flrst cerebral vesicle and its upgrowth takes the pterygo- 

 palatine rods almost from end to end, whilst the trabeculse 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, mk.). 



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, jnk.), and in the palatal view (fig. 4, mk.) near its apex ; near its apex it 

 is seen on the outside in the lateral view of the sliced head (Plate XXIX. fig. 6, mk.). 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, mk.). 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 



* Sco " On the Ilepresentativcs of the Malleus and the Incus of the Mammalia in the other Vertebrata," by 

 Professor Huxley, Proc. Zool. Sec, May 27, 18(59, pp. 391-407. 



t I was under the impression that the "internal angular process" of the Bird's mandible ("Fowl's 

 Skull," Plate lxxxi. 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 urch. 



