1 206 EMBR YOL OGY. 



connection between the intestinal cavity and the substance of this organ. It 

 originates in the inesenteric fold which connects the stomach to the vertebral 

 column (mesogastrium) (Fig. 764). 



The thyroid body is developed as a median and two lateral diverticula from 

 the ventral wall of the pharynx. The median diverticulum appears first; it com- 

 mences at the foramen caecum, between the anterior and posterior rudiments of the 

 tongue, and extends backward as a tubular duct, the ductus tliyro-glossus. The 

 lateral diverticula arise from the fourth visceral cleft and fuse with the median 



Papillary portion of tongue. 



4~Mandibular arch. 



'Hyoid arch. 



Foramen csecum. Posterior part Third arch. 



of tongue. 

 FIG. 773. Floor of mouth of au embryo slightly older than that shown in Fig. 772. X 16. (From His.) 



part to form the thyroid body. The connection of the lateral diverticula with the 

 pharynx disappears early, but the remains of the ductus thyro-glossus may persist 

 as a tube leading from the foramen caecum toward the hyoid bone, the pyramid 

 of the thyroid probably representing its lower part. 1 



The thymus is developed from bilateral diverticula, which are principally derived 

 from the third visceral cleft. It increases in size until the second year of life, 

 after which it undergoes atrophy. 



Development of the Respiratory Organs. The lungs appear somewhat later 

 than the liver. They are developed from a small median cul-de-sac or diverticu- 

 lum from the upper part of the fore-gut, immediately behind the fourth visceral 

 cleft. During the fourth week a pouch is formed on either side of the central 

 diverticulum, and opens freely through it into the fore-gut (pharynx). These 

 lateral pouches soon become subdivided the right into three and the left into two 

 parts, these subdivisions being the early indications of the lobes of the lungs 

 (Figs. 716 and 761). The two primary pouches have thus a common tube of com- 

 munication with the pharynx. This common tube becomes the larynx and trachea, 

 the latter rapidly elongating as development proceeds. The larynx first becomes 

 evident as a dilatation of the upper part of the trachea about the end of the fifth 

 week. The epiglottis is developed from the anterior or median portion of the fur- 

 cula, and the aryteno-epiglottidean folds from its lateral ridges (Fig. 772). The 

 vocal cords and ventricles of the larynx are formed about the fourth month. 



As the lungs grow backward they project into the anterior part of the coelum, 

 which becomes shut off from the rest of the body-cavity by the pericardium and 

 Diaphragm to form the pleura! cavities. 



The Diaphragm is formed in two parts : (a) ventral, (6) dorsal. The ventral 

 part appears first, and consists of a thick septum of mesoblast, the septum trans- 

 versum, which projects from the anterior and lateral walls of the embryo, and 

 which ends behind in a free edge. The sinus venosus, which receives the vitelline, 

 umbilical, and Cuvierian veins is placed originally in this septum, and into the 

 posterior part of it also the liver diverticula grow from the duodenum. The sinus 

 separates itself above from the septum, and the greater part of it is incorporated 

 with the right auricle. The liver also becomes separated from it below, except 

 where the veins pass through into the heart. The septum transversum shuts 



1 Kanthack (Journal of Anat. and PhysioL, vol. xxv., p. 155) disputed this view. He examined 

 100 subjects, 60 of which were foetuses or infants, and found that in many oases there was no trace of 

 foramen cjecum and that, when it was present, it formed a short canal near the surface and was lined 

 with stratified squamous, not columnar, epithelium. Further, after careful microscopical examina- 

 tion he found no trace of a tubular lumen in the pyramid of the thvroid body. 



