DEVELOPMENT OF THE SKULL IN THE PIG. 



295 



hammer ; the solid rod itself develops for a while, but by the time of birth has shrunk 

 into the feeble, pointed " processus gracilis." 



Fourth Arch, or Second Pastoral. — At present this arch is extremely like the one in 

 front of it (Plate XXVIII. fig. 5, and Plate XXIX. figs. 5 & 6, hy.) ; but it is flatter, 

 and the right and left bars meet more closely and at an obtuse angle ; its shoulder, 

 also, is more upturned. This arch has been cut through in the palatal vertical views 

 (Plate XXVIII. figs. 4 & 6, ^j/.); but the form of its tuberculum and capitulum are 

 best seen in the horizontal section (Plate XXIX. figs. 7-10, hy), and its shoulder or 

 tuberculum is exposed in the sliced head (Plate XXIX. fig. 6, hy.). In this latter figure 

 it is seen that the shoulder stands out like that of a Bird's rib, the head or capitular 

 portion thrusting itself as far inwards as it can on to the periotic wall. The landmarks 

 exposed in this figure are the portio dura and the top of the jugular vein (Plate XXIX. 

 fig. 6, hy., 7'',j.v.). In figs. 7, 8, & 10 of the same, Plate, the horizontal sections show 

 that the head of the hyoid growing towards the auditory mass is exactly like the head 

 of the mandibular rod. The portio dura nerve is seen both at its entrance into and its 

 exit from the facial wall in this figure, and it is of the utmost consequence to the morpho- 

 logist as being a most safe landmark. In the outer lateral view it is seen escaping 

 behind that part of the hyoid rod which becomes the " stylohyal" (Plate XXIX. fig. 6, 



In one horizontal view (Plate XXIX. fig. 7, hy., 7") its whole auditory course is seen, 

 on one side its entrance into the wall in front of the first postoral cleft, and its exit 

 behind the hyoid in the other ; the same thing is shown in Plate XXVIII. fig. 8, 7" 

 (see also Plate XXIX. figs. 8-10, 7"). In most of these figures the head and neck of 

 the hyoid are shown from above (Plate XXIX. figs. 7, 8) and from below (fig. 10, hy.) ; 

 but in another seen from above (Plate XXIX. fig. 9) the section is through the rods 

 a little lower down ; and here we get a most instructive view, the shoulder evidently 

 becoming dislocated from the neck, a process which will go on to complete separation 

 af the parts. 



Fifth Arch, or Third Postoral. — In this arch the Mammal has developed merely the 

 counterpart of the " hypobranchial" segment of the first branchial arch; it is shown 

 in a subhorizontal section in situ attached to the larynx (Plate XXIX. fig. 5, th.h., Ix.), 

 and in the diagrammatic figure (Plate XXVIII. fig. 5, th.h.) is seen beneath the audi- 

 tory sacs. In my paper on the Frog (Plate vii. p. 171) I showed how that the thyrohyals 

 were the hypobranchial remnants of the first and second branchial arches developed 

 backwards ; those of the Mammal are therefore strictly homologous with those of the 

 Frog, the latter being formed by retention of a part, which part is alone developed in 

 the former. 



Looking again at the five pairs of facial arches as a whole, we see that the only arch, 

 at present, which has developed a conjugational keystone piece is the first or trabecular: 

 this is the "prenasal rostrum" which figures so largely in my former paper on the 

 Bird's Skull. No other keystone appears afterwards in the Pig, save in the last pair ; 



