THE LUNGS. 



109 



FORMATION OF THE GLANDS OF THE ALIMENTARY CANAL. 



Under this head may be included not only those organs which are ordinarily so 

 termed, but also the lungs, and the thymus and thyroid bodies, since the early 

 development of these three organs resembles that of the true secreting glands. 



All the organs above enumerated are formed as epithelial involutions, either solid 

 at first and afterwards becoming hollowed out, or hollow from the first. As these 

 epithelial buds grow into the mesoblast, they may either bifurcate or give off lateral 

 branches, and in this manner all the ramifications of the ducts of the compound 

 racemose glands are produced. The blind extremities generally end eventually in 

 enlarged tubular or saccular dilatations. All the epithelium of the gland-saccules 

 and ducts is derived from the original epithelial sprout, while the basement 

 membranes and connective tissue and blood-vessels of the gland are derived from the 

 surrounding mesoblast. The salivary glands and most other glands of the mouth, 

 and part of the hypophysis, which must also be reckoned as a glandular development, 

 are formed in this way by involution of the buccal or stomodseal epiblast ; while the 

 lungs, liver, pancreas, thyroid, thymus, and all the small glands of the rest of the 

 alimentary canal are formed of involutions of the hypoblast. The development of 

 the teeth, which also first make their appearance as involutions of stomodeeal epiblast 

 (enamel germs), will be described after their structure has been dealt with (in the 

 part of this work which is devoted to Splanchnology). 



The lungs. The lungs begin to develope from the ventral part of the pharynx 

 at its junction with the oesophagus, in the beginning of the third day in the chick, 



Fig. 131. LUNG RUDIMENTS OP HUMAN EMBRYO OP ABOUT 4 WEEKS, SHOWING THE BUD-LIKE 



ENLARGEMENTS WHICH REPRESENT THE LOBES OF THE FUTURE LUNGS. (His. ) 



Three buds are seen on the right side, two on the left. 

 Fig. 132. LUNGS OF A HUMAN EMBRYO MORE ADVANCED IN DEVELOPMENT. (His.) 



and in the human embryo at a correspondingly early period (fig. 123, Lg). The 

 lung rudiment is at first single and median, and takes the form of an elongated 

 vertical diverticulum of the fore-gut, communicating freely with that tube, and of 

 course lined by hypoblast. Soon the diverticulum sprouts out at its lower extremity 

 in the form of two tubes which grow downwards on either side behind and on either 

 side of the heart, into a mass of mesoblastic tissue, which keeps pace in its growth 

 with the hypoblastic lung rudiment, and from which the connective tissues of the 

 future lung become ultimately developed. The extremities of the tubes in question 

 are early seen to be dilated and tabulated (fig. 131), three lobules being present on 

 the right tube, and two on the left, the division of the lungs into their lobes being 

 thus early indicated. 



The further outgrowth of the tabulations produces the rudiments of the principal branches 

 of the bronchi, one for each future pulmonary lobe, and each of these branches then gradually 



