THE ORGANS OF THE INNEK GERM-LAYER. 337 



plane to an unpaired body, which begins to degenerate 

 in the first years after birth. 



10. The thyroid gland is an unpaired organ, which arises in the 

 region of the body of the hyoid bone from either a hollow or a solid 

 outgrowth of the epithelium in the floor of the pharyngeal cavity. 



(a) The epithelial rod detaches itself from its parental tissue 



and forms lateral rods. 



(b) At a later stage these epithelial cords become separated 



into small epithelial spheres, which secrete in their 

 interiors colloid substance and are converted into 

 wholly closed glandular sacs enveloped in highly vascular 

 capsules of connective tissue. 



11. The accessory thyroid glands are paired and arise from evagi- 

 nations of the epithelium of the last pair of visceral clefts, which 

 undergo metamorphoses similar to those of the unpaired thyroid 

 gland. 



12. The accessory thyroid glands in most Vertebrates remain 

 separated from the unpaired thyroid gland by a greater (Reptiles) 

 or less (Birds) space, whereas in Mammals they appear to fuse with 

 it to form a single body. 



13. The lung is developed out of the floor of the alimentary canae 

 in the throat-region, behind the fundament of the unpaired thyroid 

 gland. 



(a) A groove-like evagination, which is constricted off" from the 

 alimentary canal as far forward as its anterior end, 

 the entrance to the larynx, becomes larynx and wind- 

 pipe. 



(6) From the posterior end of the groove there grow out two- 

 sacs, which acquire at their ends vesicular enlargements ; 

 and constitute the fundaments of the right and left 

 bronchus, together with the corresponding lung. 



(c) The want of symmetry between the right and left lung 



is early exhibited, since the right sac provides itself 

 with three vesicular lateral buds, the fundaments of the 

 three lobes, whereas the left sac forms only two buds. 



(d) The further development of the lungs allows one to dis- 



tinguish two stages, of which the first exhibits a great 

 similarity to the development of an acinous gland. In 

 the first stages the primitive pulmonary sacs increase in 

 number by constrictions and at the same time become 

 differentiated into a narrower conducting part, the 



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