172 PAEATHYKOIDS 



ance of a lymphoid organ. The epithelial cells give rise to a reticulum (Hammar ') (figs. 212, 216), 

 in the meshes of which in the cortical zone lymphoid cells collect in large numbers. The origin 

 of these lymph-cells has been much disputed, some, following Kolliker, deriving them from the 

 entodermic epithelial elements (Prenant, Beard, Maurer, Nusbaum, &c.); others, following 

 Stieda and His, regarding them as invading the rudiment from without (Gulland, Kollmann, 

 &c.). It has been proved (Hammar, Bryce, 2 Stohr 3 ) that leucocytes are already present 

 before the lymphoid transformation of the gland, so that the thymus cannot be the original 

 source of the leucocytes as suggested by Beard. Stohr in a recent publication advances the 

 thesis that the thymus is an epithelial organ throughout, and is not lymphoid, as the characters 

 of the cells would suggest. The Hassall's corpuscles are very generally, and in all probability 

 rightly, regarded as cell-nests derived from the epithelial cells, as are also various kinds of 

 minute cysts, some of them with a ciliated lining, which have been described as occurring 

 in the adult gland. 



The parathyroid bodies develop in intimate relation with the thymus and 

 lateral thyroid from the entodermic lining of the third and fourth visceral pouches 



?- - - ... ., ___ _ truncus aortic 



pericardium 



' , post .mesocar din m 



' ; ">- ..s 



'' 



V/'' / auricular portion of 



ant. wall of ^ ' ........ - heart loop 



pericardium T g\ ...-'-'"'"". 



- -------- lat. mesocardium 



\ 



. ......... --ductofCuvier 



septum' 

 transversum 



liver parenchyma .- y. 



liver divert ^= 



mouth of yoU- _ 



~" cuelom 



- 1 IS-'- umbilical vein 

 ^-.citelltnevein 



FIG. 217. THE LIVER DIVERTICULUM AND PARENCHYME WITH SEPTUM TBANSVEBSUM. 

 HUMAN EMBRYO OF 3 MM. LONG. (After His, from Kollmann.) 



of the pharynx. 4 They appear as paired epithelial buds, which soon become 

 separated from the surface from which they have sprung, and form small isolated 

 bodies of oval shape (fig. 213). The buds from the third pouches lie at first in front 

 and to the outer side of the corresponding buds from the fourth pouches, but they 

 are carried backwards with the thymus, and ultimately lie behind the others 

 (fig. 214). The epithelial cells of each bud become loosely arranged and form 

 a syncytial reticulum closely resembling that of the early thymus (fig. 216). Each 

 body has a distinct capsule, and is quite independent of the other epithelial 

 derivatives of the branchial pouches. In the early stages the parathyroids are 

 thus quite unlike the thyroid, but resemble the thymus (fig. 216). 



1 Anat. Aiizeiger. xxvii. 1905. In this paper will be found all the most important references to 

 the Hterature regarding the mammalian thymus. See also Bell, Amer. Journ. of Anat. v. 1906. 

 - Bryce, Journ. of Anat. and Physiology, xl. 1905. 



3 Stohr, Anat. Hefte, xxxi. 1906. 



4 The bud from the third pouch is often named the parathymus, while that from the fourth is called 

 the parathyroid. Here the term parathyroid is used for both bodies in view of their relations to the 

 thyroid gland in the adult. 



