DEVELOPMENT OF THE FACE. 967 



The Parts of the Face. 



The extension downwards of the lateral ventral plates of the em- 

 bryo, which, opposite the trunk, form the sides of the haemal cavity, 

 occurs also beneath the cephalic part. Here, however, where the fu- 

 ture face is developed, the haemal cavity is imperfectly closed in at the 

 sides ; for these plates, with their covering and lining membranes from 

 the external and internal, or epidermic and intestinal germinal layers, 

 split, on each side, into four processes or lobes, with little clefts be- 

 tween them, forming the so-called visceral or branchial arches, and the 

 visceral or branchial clefts. The term branchial is applied to these 

 arches, because the permanent gills or branchiae of the Fish, and the 

 corresponding temporary gills of the Amphibia, are developed from 

 homologous parts ; but in the embryos of the Reptile, Bird, and Mam- 

 mal, these arches give rise, through very early metamorphoses, to 

 other organs. Within them, minute, but important vessels, as will be 

 hereafter seen, are temporarily present. Gills are never developed 

 on them, and they never exercise a respiratory function. In these 

 three last-named Classes, the allantois, a part not present in Amphibia 

 and Fishes, is the embryonic respiratory organ. 



The first branchial cleft, above the first arch, sometimes named the 

 maxillary cleft, forms the cavities of the mouth and nose ; these are 

 originally conjoined, but subsequently become separated, by the growth 

 of the upper jaw, from the substance of the first arch, between the 

 nasal cavities above and the mouth below. The nasal walls and septum 

 grow downwards from the cranium, whilst the upper jaw and palate are 

 developed transversely from the face, to meet them. From the posterior 

 part of the second branchial cleft, between the first and second 

 branchial arches, are formed, the cavity of the tympanum, which at 

 first contains soft connective tissue, the Eustachian tube, which is also 

 at first filled with a similar tissue, the membrana tympani, and the 

 external auditory meatus and its appendages. The auricle commences 

 as a little ring around the margin of the meatus. The third and fourth 

 branchial clefts completely close up, andf disappear very early. 



Within the branchial arches, little cartilaginous plates are soon de- 

 veloped. From the upper edge of the first of these arches a process 

 is formed, named the maxillary lobe, from which the upper jaw is de- 

 veloped, together with the whole side of the face, including the inter- 

 nal pterygoid process and the palate-bone. The malar bones and 

 lachrymal bones are formed as opercular bones. The first arch also 

 gives origin, by another process, to the rudimentary lower jaw, and 

 likewise, it is said, to the tongue. From the cranium, a median pro- 

 cess, known as the frontal process, descends in the middle line of the 

 face ; and with this, an external and internal nasal process are also 

 connected. These, by their junctions, form the walls and partition 

 of the nasal fossae, and the centre of the upper lip. In this latter 

 part, the intermaxillary bones, which carry the upper incisor teeth, 

 are independent formations. The lachrymal duct is a fissure which 

 remains partly open, between the external cranial nasal process and 



