DEVELOPMENT OF THE TONGUE. 



103 



The furcula gives rise to the epiglottis in front (above) the aryepiglottic folds on 

 either side, and the arytenoid cartilages behind (below) ; the median groove in it leads 

 to the entrance of the larynx. 



Laterally the 2nd arch passes into and forms the palato-glossal arch, and in the visceral 

 cleft behind this the tonsil develops ; but the 3rd arch does not form the palato-pharyn^eal 

 arch ; this is developed from the palatine outgrowths of the maxillary processes 



The visceral arches were first described by Rathke, in 1825. They are often dis- 

 tinguished as the post-luccal visceral arches ; certain parts in front of (above) the mouth 



B 



Fig. 121. SIMILAR VIEWS OF THE SAMK PARTS IN OLDER EMBRYOS. (His.) A. ^ B. ^ 



T, tuberculum impar. 



being considered by some morphologists to represent pree-buccal arches. The visceral clefts, 

 lying between the arches are, as has been stated, four in number. The first is often known as 

 the hyomandilular cleft : it is this one which is concerned with the formation of the 



Fig. 122. SIMILAR VIEW IN A CONSIDERABLY OLDER 



EMBRYO, BUT LESS MAGNIFIED. (His. ) 



Eustachian tube and middle ear as already de- 

 scribed. The three remaining 1 clefts, which repre- 

 sent gill-slits of fishes and amphibia, appear, from 

 the results of recent observations, to be closed in 

 amniotic vertebrates at all periods of foetal life 

 (see note on previous page). In some fishes the 

 branchial arches and clefts are more numerous 

 than in other vertebrates, and in a few the hyoid 

 arch also develops a gill. 



Through each of the visceral arches an arterial 

 arch derived from the aortic bulb passes from 

 front to back reuniting dorsally in front of 



the notochord to form the aorta. In branchiate vertebrates, branches of these vessels are 

 distributed to the gills. Cartilaginous bars pass, in most vertebrates, from the base of the 

 skull into each visceral arch, and ossification occurring in or around them, form definite parts 

 of the skeleton as will be afterwards described. In man and mammals these cartilaginous 

 bars are only found in the first three visceral arches, unless the thyroid cartilage is to be 

 regarded as representing the anterior (ventral) ends of the bar of the 4th arch (Callender). 

 The fourth and fifth visceral arches may be considered as belonging to the neck rather 

 than to the head, and the congenital fissures of the neck which sometimes occur as a mal- 

 formation, and which usually open externally far down in the cervical region, have been 

 regarded as due to persistence of one. or more of the branchial clefts, shifted in position by the 

 cervical elongation which takes place in later embryonic life. 



(Esophagus, stomach, and intestines. Immediately behind the pharynx, the 

 fore-gut contracts again to form the oesophagus, which, in the early embryo, corres- 

 ponding with the imperfect development of the neck, is very short (figs. 123, 125, A) 



