Till GERM-LAY KK. 317 



The fin-tin / /</</,,/_// <>f the. thiiinus in M ''<>. recognition 



ntds, one of progressive and one of regressive development. 



The lirst period extends into the second year after birth. The 

 tliymus of the right side and that of the left move in their growth 

 do>e together into the median flane and here t'u-e into an unpaired, 

 lohed orpin. whoso double origin is to be recognised only by the fact 

 that tin- uriMn i> ordinarily composed of lateral ha 1 by 



connective ti. ue. It lies in front of [ventral to] the pericardium and 

 the la me blood-vessels beneath the breastbone, and is often elongated 

 into two hurn.N wliich extend upwards to the thyroid gland. 



The second period exhibits the organ undergoing regressive meta- 

 morpho>is, wliich usually leads to its total disappearance, the par- 

 ticulars of which can be learned from the text-books of Histology. 



(2) The Thyroid Gland 



is found on the anterior surface of the neck, and appears to be 

 developed in almost all classes of Vertebrates in a tolerably uniform, 

 typical manner from an unpaired and a paired evagination of the 

 pbaryngeal epithelium. We must therefore distinguish unpaired and 

 paired fundaments of the thyroid gland. 



The unpaired fundament has been longest known. There is not 

 a sinirle class of Vertebrates in which it is wanting, as has been 

 established especially by the investigations of W. M i LLER. It 

 appears to bo an organ of very ancient origin, which shows relation- 

 ship to the hypobranchial furrow of Amphioxus and the Tunica 



DOHRN has opposed this hypothesis and has expressed the view, which is also 

 .-hared by others, but which lacks proof, that the thyroid gland is the remnant 

 of a lost gill-cleft of the Vertebrates. 



The unpaired thyroid ^land arises as a small evaluation of the 

 epithelium of the front wall of the throat in the median plane and 

 in the vicinity of the second visceral arch. Then it detaches itself 

 completely from its place of origin, and is converted either into a 

 solid spheroidal body (Selachians, Teleo>N, Amphibia, etc.) or into an 

 epithelial vesicle having a small cavity (Birds, Mammals, Man, etc.). 

 The vehicle subsequently loses its cavity. 



In Man the development of the unpaired part of the thyroid gland is relau-d 

 to the formation of the root of the tongue, a- 11 is stairs in 1. ations 



of human embryos. The previously described ridirrs lyinj: on the floor of the 

 threat -cavity in the vicinity of the second and third visceral arches, which unite 

 in the median plane to form the root of the tongue, surround a deep depre.- 



