LARYNX 



167 



rudimentary fifth branchial arches (Kallius), but they soon lose any apparent 

 relation to the branchial region by enlarging dorsally and by growing forwards till 

 they reach almost to the level of the second visceral pouches. They are connected 

 with the floor of the pharynx by two folds which run into the transverse ridge 

 intervening between the ventral ends of the third arches. This ridge, and 

 the lateral folds, form the furcula of His, and, as already mentioned, the 

 epiglottis appears as a fold on the transverse ridge, while the lateral bands form the 

 aryteno-epiglottidean folds. On the anterior margin of the arytenoid mass, 

 external to its pointed end, a swelling, which appears very early, represents the 

 rudiment of the cartilage of Santorini. At an early stage, after the trachea and 

 oesophagus have separated, the slit-like cavity between the swellings is for a time 

 partially obliterated by the cohesion of the opposed epithelial surfaces. 



The connective tissue round the now slit-like glottis becomes condensed into 

 chondroblast. In this precartilaginous stage the rudiments of the arytenoids, the 



rvtrnoi<l 



\yoid cartilage II. 



FIG. 210. TRANSVERSE SECTION OF THE PHARYNX AND LARYNX OF A HUMAN EMBRYO OF 15'5 MM. 



Photograph. (T. H. Bryce.) 



c.a., c.a., carotid arteries. 



cricoid, and cartilages of the trachea are continuous laterally. The cricoid 

 chondrifies by two centres, one on each side. These unite ventrally, but the ring 

 remains open behind for a time, until completed by the extension of chondrification 

 into the dorsal plate. The tracheal rings develop in the same way, but remain 

 incomplete. The arytenoids are at first continuous with the cricoid by fibrous 

 tissue ; the cartilages of Santorini are portions of the arytenoids segmented off. 

 The epiglottis chondrifies relatively late, and is at first continuous behind with the 

 cartilages of Wrisberg, which are thus derivatives of the epiglottis (Goppert). The 

 thyroid is laid down in the form of two lateral plates which are united ventrally 

 by an intermediate nodule of cartilage (Nicolas). Each plate chondrifies from two 

 centres, an anterior and a posterior (Kallius), possibly representing separate bars 

 seen in Echidna, derived from the third and fourth visceral arches (Goppert). 



