260 



NORMAL HISTOLOGY. 



FIG. 296. 



developing system of primary air-tubes and alveoli that its relative 

 quantity is greatly reduced, since it eventually is limited to the 

 connective-tissue framework of the organ. 

 The appearance of the blood-vessels occurs at 

 a later period. The derivation of the greater 

 part of the digestive and of the respiratory 

 tract is identical, namely, the epithelial 

 structures from the entoderm and the re- 

 maining tissues from the mesoderm. 



The development of the thyroid body 

 includes the history of two structures which 

 originate independently, but which after a short 

 time in man and other mammals become 

 blended to constitute a single organ : in many 

 animals, however, the mesial and lateral thy- 

 roid areas produce organs which permanently 

 remain distinct. 



The middle thyroid area, from which originates the true thyroid 

 body, appears as a ventral outgrowth from the entodermic lining 

 of the primitive pharynx at a position corresponding approxi- 

 mately with the second visceral arch. The mesial outgrowth rapidly 

 elongates, and after a time usually loses its attachment with the 



Portion of sagittal section 

 of twelve-day rabbit em- 

 bryo, exhibiting mesial thy- 

 roid area as epithelial out- 

 growth (/) still connected 

 with pharyngeal entoderm 

 (e) ; nt, surrounding meso- 

 derm. 



FIG. 297. 



pharyngeal epithelium. The entodermic mass, 

 gradually leaves the primitive pharynx and as- 

 sumes a close relation with the paired lateral 

 thyroid areas, with which it eventually fuses. 



The lateral developmental areas of the 

 thyroid body appear as ventral outgrowths 

 from the entodermic lining of the fourth inner 

 visceral furrow on either side. The epithelial 

 evaginations become elongated cylindrical 

 masses, which undergo active proliferation and 

 extend their bulk as branching cords ; where 

 these are at first solid they subsequently obtain 

 a lumen, and for a time present the character of 

 tubular glands. The later changes include the 

 approximation and final fusion of the two lateral 

 and the single mesial areas to form the thyroid 

 body of the mammalian type. The disappear- 

 ance of excretory ducts and the ingrowth 

 of the surrounding mesoderm result in the division of the organ into 

 lobules and the isolation of the imperfectly-developed acini. Disten- 

 tion of the latter by accumulations of colloid material follows the 

 activity of the secreting cells within the ductless alveoli. 



Portion of section of four- 

 teen-day rabbit embryo, in- 

 cluding lateral thyroid area 

 (t) which is stilf attached 

 to fourth inner pharyngeal 

 furrow (/) ; m, surrounding 

 mesoderm. 



