958 



OF GENERATION EMBRYONIC DEVELOPMENT. 



The lower jaw thus stands in the same relation to Meckel's Cartilage that 

 the opercular bones do to the primitive cranial bones; and it originally con- 

 sists of two halves, united by a kind of synchoudrosis, which does not ossify 

 till after birth. The malleus, which is the homologue of the quadrate bone 

 of Birds, ossifies in the fourth mouth from the surface inwards, whilst 

 Meckel's cartilage dies away, except near its root, which ossifies into the 

 processus gracilis, or long process of the malleus. The superior maxillary 



FIG. 359. 



FIG. 360. 



FIG. 361. 



x ,' -r 



-</>'' f- 



,cL -'/;.- 



FIG. 359. Human Embryo at the end of the third week : 1, 2, 3, 4, the visceral arches ; a, the maxil- 

 lary process ; b, the eye. 



FIG. 360. Human Embryo at the close of the fifth week : 1 and 2, the first two visceral arches ; a, the 

 superior maxillary process ; b, the eye ; c, the lateral naso-frontal process ; nf, the naso-froutal process ; 

 I, the tongue. 



FIG. 361. Diagram of the first and second post-stomal (visceral) arches of Human Embryo: a, incus; 

 the line/points to the apex of the second arch, containing the os orbiculare ; 6, malleus ; the tip of the 

 long process is the apex of the first visceral arch ; c, Meckel's cartilage ; d, dentary ; e, stapes; /, os 

 orbicularo; ;/, upper hyoid cartilage; between g and e is the rudiment of the stapedius muscle ; m, 

 middle hyoid cartilage; i, rudiment of the cornu minus of os hyoides; k, basi-hyal ; I, tympanic 

 annulus. 



process (a, Fig. 360) of the first visceral arch furnishes near its base the 

 Pterygoid (k, Fig. 361) and distally the Palatine (i) bones, which begin to 

 ossify in nascent or simple cartilage at about the close of the second month. 

 The superior maxillary and zygomatic bones each ossify from one centre, at 

 the end of the second month ; and these, together with the lachrymal bones, 

 are to be regarded as opercular bones. With regard to the intermaxillary 

 bone, it is probable that this is developed from two ossific centres in the mem- 

 brane, independently of the superior maxilla, though they join with it about 

 the middle of the third month. 1 Kollman, 2 however, describes it as originat- 

 ing from the frontal process by two halves, which subsequently unite. The 

 Vomer bears the same relation to the base of the septum uasale; it appears 

 in the third month, aud has then the form of a semi-canal. At first, the 

 mouth is a wide cavity, which is subsequently divided into a respiratory and 

 digestive cavity by the lateral growth of the superior maxillary processes of 

 the first visceral arch ; thus constituting partitions on either side, called 

 Palate plates, which after the eighth week begin to coalesce from before 

 backward. An arrest of development at this period produces the deformity 

 termed Cleft palate. The second visceral or hyoid arch is originally in con- 

 nection with the base of the skull, near the posterior sphenoid bone, but 



1 See Cullender, Pliil. Trans., 1809, pt. i, pp. 163-172, plates 13 and 14. 



2 Zeits. f. Biolog., Bd. iv, 1868, p. 274. 



