THE ORGANS OF THE INNER GERM-LAYER. 317 



The further history of the thymus in Man permits the recognition 

 of two periods, one of progressive and one of regressive development. 



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

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

 close together into the median plane and here fuse into an unpaired, 

 lobed organ, whose double origin is to ba recognised only by the fact 

 that the organ is ordinarily composed of lateral halves separated by 

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

 the large blood-vessels beneath the breastbone, and is often elongated 

 into two horns which extend upwards to the thyroid gland. 



The second period exhibits the organ undergoing regressive meta- 

 morphosis, which 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 

 pharyngeal epithelium. We must therefore distinguish unpaired and 

 paired fundaments of the thyroid gland. 



The unpaired fundament has been longest known. There is not 

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

 established especially by the investigations of W. MULLER. It 

 appears to be an organ of very ancient origin, which shoAvs relation- 

 ship to the hypobranchial furrow of Amphioxus and the Tunicates. 



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

 shared by others, but which lacks proof, that the thyroid gland is the remnant 

 of a lost gill-cleft of the Vertebrates. 



The unpaired thyroid gland arises as a small evagination 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, Teleosts, Amphibia, etc.) or into an 

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

 The vesicle subsequently loses its cavity. 



In Man the development of the unpaired part of the thyroid gland is related 

 to the formation of the root of the tongue, as His states in his investigations 

 of human embryos. The previously described ridges lying on the floor of the 

 throat-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 depression, 



