738 



DEVELOrMENT OF THE HEAD. 



2. Subcranial, Facial, or Pharyngeal Plates or Arches. — 



In man, and all vertebrates, there are developed below and on the 

 sides of the cranial part of the head, a series of processes or bars in 

 pairs, which contribute to the formation of the subcranial structures 

 constituting the face and jaws, and the hyoid and other parts inter- 

 vening between the head and trunk. These bars first received attention 



Fig. 539. 



Fig. 539. — Outlines showing the early changes in the form of the Head op 

 THE Human Embryo. 



A, profile view of tlie head and fore i)art of the body of an embryo of about four weeks 

 (from nature, '^^) : the five primary divisions of the brain are shown, together with the 

 primai-y olfactory and optic depressions, and a, the auditory vesicle ; 1, marks the man- 

 dibular plate, and behind this are seen the three following plates with the corresponding 

 pharyngeal clefts. B, from an embryo of about six weeks (from Ecker, f ) : the cerebral 

 hemispheres have become enlarged and begin to spread laterally ; 1, the lower jaw ; 1', 

 the first pharyngeal cleft, now widening at the dorsal end, where it forms the meatus 

 externus ; the second cleft is still visible, but the third and fourth clefts are closed and 

 *he corresponding plates have nearly disappeared. C, from a human fojtus of nine weeks 

 (from nature, \) ; the features of the face are now roughly formed ; the first pharyngeal 

 ■cleft is now undergoing conversion into the meatus, and the auricle is beginning to rise at 

 its outer border. 



from their discovery by Eathke in 182G, published in the Isis of that 

 year, and were named by him the branchial arches, from the relation 

 ■which some of them bear to the gill bars of branchiate vertebrates. 

 Their nature and transformations were fully investigated by Eeichert 

 in 1837 (Miiller's Archiv, 1837). From later researches it appears that 

 other processes, with somewhat similar relations to the cranium, occurring 

 further forward, may be associated with those described as branchial by 

 Eathke, and it will be expedient therefore to describe the whole of the sub- 

 cranial outgrowths together at this place. In this the views of Huxley 

 and Parker will be chiefly followed. (See "On the Stracture and Develop- 

 ment of the Skull in the Pig," by W. K, Parker, in Trans. Eoy, Soc. 

 1873, p. 289 ; Huxley, in Elements of Compai\ Anat., 1864, and Manual 

 of Compar. Anat., 1871 ; also Gegenbaur, Das Kopfskelet, &c., 1872). 

 According to these views the parts of the head situated in fi*ont of and 

 above the future mouth, are formed from two pairs of plates, w^hich may 

 thence be called preoral, in one pair of which the bars are the same 

 "with the trahecukc cranii of Eathke surrounding the pituitary gland, 

 and are the basis of formation of the pre-sphenoid, ethmoid, nasal, and 

 pre-maxillary portions of the skull, while in the other pair, consisting in 

 each of a deeper and a superficial part, the bars form the foundation 



