THE FOREGUT 203 



epithelium liquefaction while erosion of the colloid and follicular col- 

 lapse occur. At this time the genio-hyoid, hypoglossus, and sterno- 

 hyoid muscles are in close proximity to the thyroid, a situation which 

 does not persist to the adult. The gland shows hyperactivity during 

 metamorphosis with relative inactivity at the completion of the 

 metamorphic process. Its activity during these phases of development 

 is closely correlated with the development and function of the pitui- 

 tary gland, particularly of the basophilic cells in the pituitary gland. 

 In later stages it may be seen attached to the ventral aspect of the 

 hypoglossus muscle. The fully formed thyroid gland consists of sepa- 

 rate follicles, each made up of a single circular layer of cuboidal 

 (endodermal) epithelial cells, in the center of which is a lumen filled 

 with a colloidal mass. 



The Tongue. 



The tongue appears just before metamorphosis and is indicated as 

 an elevation in the anterior floor of the pharynx, just posterior to the 

 site of origin of the thyroid gland. Anterior to this the pharyngeal floor 

 is depressed and glandular, but during metamorphosis this glandular 

 area becomes the free anterior tip of the tongue. 



The Lungs. 



Shortly after the time of hatching, when the larva measures about 

 6 mm. in length, there appear bifurcating but solid cell proliferations 

 from the pharyngeal endoderm just behind the developing heart. 

 These soon develop into paired saccular evaginations, directed pos- 

 teriorly. These lung buds arise from the median ventral floor of the 

 pharynx at about the level of the rudimentary sixth visceral (fifth 

 branchial) pouch. The single short tubular connection of the lung buds 

 opens into the foregut through the glottis. The connecting column of 

 ceHs which has acquired a lumen, from which the lung buds arise, 

 will become the trachea. At the level of the glottis there is a short 

 transverse chamber known as the laryngeal chamber. The more pos- 

 terior bi-lobed mass eventually will open up as the primary bronchi 

 or lung buds. In the 1 1 mm. stage each lung bud will be surrounded by 

 peritoneal epithelium and will be invested with splanchnic mesen- 

 chyme. Later, each lung will become an ovoid, thin-walled, and 

 slightly alveolated sac, lined with endodermal squamous epithelium. 

 Outside of this epithelium, and constituting the substance of the lung, 



