224 



HISTOLOGY 



original diverticulum. Each lobule consists of a pale medulla, extend- 

 ing from the cord, and a darker peripheral cortex (Figs. 212 and 213). 

 The entire structure somewhat resembles a lymph gland, from which, 

 however, germinal centers are absent. It might be inferred that lym- 

 phoid tissue had developed in the mesenchyma surrounding the diver- 

 ticulum, in the same way that such tissue forms about the tonsillar 

 pits, but careful study has shown that the thymus is largely of ento- 

 dermal origin. Whether the cells of its cortex, which closely resemble 

 lymphocytes, are true lymphocytes or "deceptively similar epithelial 

 cells" has not been determined. 



Vein. 



Connective tissue. 



Thymic corpuscle. 



\ 



Entering Medullary 

 leucocytes. substance. 



Cortical 

 substance. 

 FIG. 214. PART OF A SECTION OF THE THYMUS OF A CHILD AT BIRTH. X 50. 



According to Bell (Amer. Journ. Anat., 1905, vol. 5, pp. 29-62) the thymus is at 

 first a compact mass of entodermal cells. By vacuolization the cells form a reticulum, 

 and certain of them become lymphocytes. The lymphocytes pass 'into the cortex 

 where they are most abundant, and enter the vessels. The lymphoid transformation 

 of the thymus "is noticeable in pigs of 3.5 cm. and is well advanced at 4.5 cm.' Thus 

 lymphocytes appear in the thymus at about the time that lymph glands develop. 

 The first indication of lymph glands was found by Miss Sabin in pig embryos of 3 cm. 



That the thymus cells are lymphocytes, however, is denied by Stohr, who regards 

 the cortex as composed of a network of stellate epithelial cells, containing in its meshes 



