36 GENERAL EMBRYOLOGY. 
of the body and the great cornu of the hyoid bone on its own side. — Its blood- 
vessel, the third cephalic aortic arch, becomes the lower part of the stem of the 
internal carotid artery. 
The fourth and fifth visceral arches also enter into the formation of the neck, 
but their exact limits in the adult cannot be defined. Of the upper sections of their 
cartilaginous bars no trace remains in the adult, but their lower portions are 
believed to enter into the formation of the thyroid cartilage of the larynx. The 
blood-vessel of the fourth arch on the right side becomes part of the right sub- 
clavian artery, that on the left side is converted into the arch of the aorta. The 
vessels of the fifth arches form portions of the pulmonary arteries, and that on the 
left side forms also the ductus arteriosus. 
Further Development of the Ventral Wall of the Fore-gut in the region of 
the Furcula.—The sinus arcuatus which surrounds the furcula disappears to a 
great extent as development proceeds, but certain parts of 1t remain and are recog- 
nisable in the adult. The anterior portion immediately in front of the furcula is 
divided into two parts as the lower ends of the second and third arches of the two 
sides converge and fuse in the ventral wall of the pharyngeal portion of the 
foregut; the middle portion of the sinus, in front of the tranverse bar formed by 
this fusion, persists im the adult as the foramen cecum of the tongue, and at a very 
early period a diverticulum grows backwards from it in the floor of the pharynx, 
dorsal to the cartilage bars which form the hyoid bone, but ventral to the rudi- 
ments of the thyroid cartilage. This diverticulum is the thyro-glossal duct. As 
soon as it reaches the level of the fourth visceral clefts it enlarges, unites with the 
diverticula from those clefts which form the lateral lobes of the thyroid body, and 
is itself converted into the isthmus of the thyroid, its pyramidal process and the 
thyro-glossal duct or the fibrous cord into which that duct becomes converted in 
the adult. Occasionally the thyro-glossal duct is not wholly transformed into a 
fibrous cord but portions of it remain in the form of isolated vesicles, hned with 
columnar or cubical epithelium, or as cords of cells, and these occasionally undergo 
abnormal development, forming tumours at the base of the tongue or in the upper 
part of the neck. 
The portion of the sinus arcuatus which hes behind the conjoined lower 
extremities of the second and third arches of opposite sides, and in front of the 
furcula, persists in a modified form in the adult, and is recognisable as glosso- 
epiglottidean pouches or valleculee at the base of the tongue. 
The furcula and the groove in the ventral wall of the fore- gut, which it embraces 
antero-laterally, are both of considerable importance. The “anterior part of the 
furcula is situated in the ventral wall of the pharyngeal portion of the fore-gut, but 
its backward prolongations and the furrow between them lie in what may be 
termed the intermediate part of the fore-gut, that is, in that part of the fore-gut 
which intervenes between the pharyngeal and stomach regions. Gradually the 
furrow deepens, and its posterior extremity dilates on each side. Afterwards the 
margins of the furrow coalesce from behind forwards, and in this manner the 
cavity of the furrow is separated from the fore-gut, its walls are converted into the 
trachea and the lower part of the larynx, whilst the diverticula which are projected 
from its posterior end, form the rudiments of the bronchi. The fusion of the 
margin of the furrow ceases a short distance behind its anterior extremity, which 
latter persists as the superior aperture of the larynx. The anterior part of the 
furcula, which bounds this aperture in front, becomes the epiglottis, and its lateral 
extensions, which form the margins of the aperture, are converted into the aryteno- 
epiglottidean folds in the substance of which the arytenoid cartilages and the 
cartilages of Santorini and Wrisberg (cuneiform cartilages) are formed. 
DEVELOPMENT OF THE MOUTH AND THE NOSE. 
The nose is formed entirely from the stomatodeum. The mouth has a double 
origin; the roof and fore part, including the teeth, are developed from the stomato- 
dieum, whilst the floor and the tongue are developed from the pharyngeal portion of 
the fore-gut. 
