610 EMBRYOLOGY. 



carry, in the mucous membrane investing them, the teeth of the 

 jaws. The two mandibular elements are united to each other in 

 the median plane by means of a mass of tense connective tissue. 

 The following visceral arches, on the contrary, are alike in having 

 their lateral halves, which are divided into several pieces, joined 

 ventrally by means of an unpaired connecting piece, the copula, in 

 a manner similar to that in which the ribs are united by the sternum. 

 The pieces of the hyoid arch are designated, in sequence from the 

 dorsal to the ventral side, hyomandibular, hyoid, and (the copula) 

 OS entoglossum. 



In Mammals and Man (figs. 154, 157) structures similar to those 

 of the Selachians are formed in the membranous stage, but sub- 

 sequently they are only in part converted into cartilaginous pieces, 

 which in turn never acquire a great size, having meantime lost 

 their original function. They help to form the facial part of the 

 head-skeleton, and have already been treated of partially in 

 previous chapters — in the discussion of the head-gut and of the 

 organ of smell. I am therefore compelled for the sake of continuity 

 to repeat much that has already been presented concerning the 

 visceral skeleton. 



In very young human and mammalian embryos the mouth-opening 

 is bounded on the sides and below by the paired maxillary and 

 mandibular processes (iig. 156, compare p. 284). The former are 

 widely separated from each other, because the unpaired frontal 

 process, in the form of a broad, rounded projection, is at first 

 inserted from above between them. Afterwards this projection 

 becomes divided by the development, on its rounded surface, of the 

 two nasal pits with the nasal grooves leading down to the upper 

 margin of the mouth (compare p. 513); it is then divided into the 

 outer and inner nasal processes. The former are separated from the 

 maxillary process by a groove, which runs from the eye to the nasal 

 f uri'ow, and is the first fundament of the lachrymal duct. 



Behind the first visceral arch comes the hyoid arch (figs. 157, 158 

 zb), the two being separated by a small visceral cleft, which becomes 

 the tympanic cavity and Eustachian tube. This is followed by three 

 additional visceral arches with three visceral furrows (or clefts), 

 which are of only short duration. 



During a later stage fusions take place between the processes that 

 surround the oral opening (fig. 332). 



The maxillary processes, by growing farther inward, meet the 

 inner nasal processes, fuse with them, and produce a continuous 



