328 THE DUCTLESS GLANDS 



thymus. From a phylogenetic standpoint the ectodermal 

 and the entodermal thymus representatives must be regarded 

 as two distinct organs, which, through a parallelism in develop- 

 ment, have acquired a similar structure. 



There are three types of thymus in mammals : 



1. A purely entodermal thymus. This is found in the 

 human subject and in the rabbit. 



2. A purely ectodermal thymus. This is found in the 

 mole. 



3. A mixed entodermal and ectodermal thymus. This 

 condition is found in the pig and the guinea-pig. 



While in crocodiles and birds the thymus is situated in the 

 neck, in mammals it is for the most part situated in the thorax. 

 But in some mammals there is a cervical portion as well 

 as a thoracic portion, while, again, in some species, such as 

 the guinea-pig, the structure is entirely cervical. How far 

 the distinction between an entodermal and an ectodermal 

 thymus corresponds to the cervical and thoracic representatives 

 of the gland is not known. But it is stated that the cervical 

 thymus of the guinea-pig is entirely entodermal, being derived 

 from the third cleft, and corresponds to the human gland, 

 which, however, is thoracic. 



This acceptance of a dual origin of the mammalian thymus 

 will necessitate a reinvestigation of the development of the 

 organ throughout vertebrates. But it must be borne in 

 mind that a definite statement as to whether a derivative 

 of a gill-cleft is ectodermal or entodermal in origin is often 

 a matter of extreme difficulty. 



The human thymus is derived from the third visceral pouch, 

 but it is not yet decided as to whether there is an accessory 

 rudiment from the fourth pouch. The thymus is thus in 

 its- first origin bilateral. A pocket develops from the third 

 cleft on each side, and extends itself as a thick-walled tubular 

 prolongation along the carotid artery. The pocket persists 

 as the ' c thymus vesicle ' ' in the proximal section of each 

 rudiment. From the lower end of the tube solid epithelial 

 buds are given off, and from these lateral buds again come 

 off, so that this part of the gland acquires a ramified lobular 

 appearance like an acinous gland. The acini, however, are 

 solid. The two rudiments are brought into close contact 

 with one another in front of the trachea, and unite to form 



