THYMUS IN THE MAESITPIALS. 553 



Unlike the 3rd, however, these parts differentiate along distinct 

 lines ; the ventral pocket loses its lumen and takes on the 

 characters of the median thyroid. The dorsal part comes to 

 resemble the thymus. The portion of the pouch carrying both 

 structures comes to be separated from the pharynx. They part 

 company, and the ventral portion enters into the composition of 

 the definitive thyroid. The dorsal thickening finally becomes 

 lodged within the capsule of the definitive thyroid at the posterior 

 border of the latter. It still retains, however, its thymus-like 

 structure. 



It has been shown within comparatively recent times that 

 various structures are constantly met with in association with 

 the thyroid. These were first definitely described by Sandstrom 

 in 1880, and termed by him " parathyroid glands." Sandstrom 

 believed these bodies to consist of portions of thyroid tissue 

 retaining more or less their embryonic structure. It was 

 pointed out in 1894 by Prenant (6), and in 1895 by Kohn (7), that 

 these bodies were distinct in structure from the thyroid, and had 

 in fact a morphological individuality of their own. Kohn further 

 proposed the terms " internal and external epithelial bodies " as 

 substitutes for Sandstrom's original names, preferring these 

 since they presupposed no morphological relationship to the 

 thyroid. Kohn showed that these bodies vary somewhat in 

 minute structure and in position with respect to the lateral 

 lobes of the thyroid, and that the external body is always either 

 free from the latter organ or incompletely buried in its substance, 

 while, on the other hand, the internal capsule is always completely 

 included in the tissue of the thyroid. In 1896 the important 

 memoir of Simon appeared, and within the last few months the 

 papers of Welsh (4). 



Simon (8) took exception to some of de Meuron's conclusions 

 as to the development of the derivatives of the 4th branchial 

 pouch. He contended that the account of the latter author of the 

 origin of the " glandule thyroVdienne " was probably erroneous. 

 Instead of this body originating in a dorsal thickening of the 

 wall of the 4th pouch as de Meuron supposed, it according to 

 Simon develops as a differentiation of the diverticulum of that 

 which forms the lateral anlage of the thyroid. Further, Simon 

 emphasizes the fact, already insisted on by de Meuron and 

 others, that the greater part of the definitive thyroid is formed 



