DEVELOPMENT OF THE SKULL IN THE PIG. 



317 



The fore part of the curious thick leaf of cartilage which grows out of the ali- and basi- 

 sphenoid is here cut through ; it is the conjugating process between the first and second 

 preoral arches. Here the zygomatic process of the squamosal (z.sq.) overlaps the jugal 

 (j.), and here the cartilaginous part of the lower jaw is nearly at its thickest ; in the root 

 of the tongue the stylohyals are severed at their junction with the ceratohyals [c.lnj.). 



The ninth section (Plate XXXIII. fig. 7) has been made through the orbito-sphenoid 

 (0.5.) close in front of the osseous nucleus (see Plate XXXIV. fig. 6, o.s.) ; it has passed 

 down through the low part of the presphenoid, where it is crossed by the optic nerve, 

 and where its territory ends and that of the basisphenoid [h.s.) begins. 



Hence in this figure we have the alisphenoids (al.s.) cut through beneath the orbital 

 AAdngs. At this point the hooked coronoid process is severed at its apex (cr.) ; and 

 outside this is the "squamosal" (sq.), with its articular cartilage and "meniscus." 

 Below these the articular and angular part of the lower jaw is shown ; it is one mass of 

 hyaline cartilage. Meckel's cartilage and the stylohyal are also cut through. 



Part of this section is shown from the riffht side (Plate XXXIII. fig. 8), magnified 

 twice as much. Here the three cartilages that form the mandibular hinge are all 

 secondary, and, like the outer ear, suggest a reversion to the labial and opercular 

 cartilages of the Shark. 



The tenth section (Plate XXXIII. fig. 9) is through the mallcal portion of the first 

 preoral arch (ml.), the front of the tegmen tympani (f.ti/.), the long, overlapping, pos- 

 terior angle of the orbito-sphenoid (o.s.), the cochlea (c/.), the stylohyal (st.h), and the 

 " occipito-sphenoidal synchondrosis" with its enclosed notochord (b.o., nc). 



The eleventh section (Plate- XXXIII. fig. 10) displays the ampulla of the anterior 

 canal (a. sc.), the general cavity of the labyrinth (vb., cL), and the tegmen tympani 

 (t.ty.), with a bony eave formed by the squamosal (sq.) and roofing over the body of 

 the incus (/), The incus is seen turning up its "long crus" and spreading its orbi- 

 cular apex over the top of the stapes (d.), which has been cut through from top to 

 bottom, exposing the foot-hole. Below the stapes is the cartilaginous wall of the pro- 

 montory {])r.), and outside this is a continuation of the tympanic cavity, in the outer 

 wall of which is the stylohyal [ty., st.h.). 



Fifth Stage. — Embryo Pig, 3 inches long. 



This is merely introduced to show the ossification of the "alisphenoids" (Plate 

 XXXIII. fig. 11, al.s.), which had not begun in the last, whilst in the next they are 

 one solid mass of bone with the basisphenoid [h.s.). Here it will be seen that the 

 posterior sphenoid is much simpler in the Pig than in Man ; in the Sheep at this stage 

 I find it simpler still, not being able to discover any median centre ; but the two alisphe- 

 noids are to be seen becoming pointed below, and growing towards each other ; here, 

 then, there appears to be no median piece, and thus the postsphenoid is like the ante- 

 rior region, in which the orbito-sphenoids themselves fill in the mid region with bone. 

 The contrary takes place in the Rodents ; and especially in the Guineapig ( Cawf « aperea) 



