3IO HISTOLOGY 



This reticulum is the basis of structure in the thymus tissue of young 

 animals, and in its meshes lie the other cells. These are the thymic 

 lymphocytes, the myoid cells, the HassalVs cells, and the several types of 

 ciliated or bordered cells found more commonly in the thymus of lower 

 vertebrates but also seen in the mammals. 



The thymic lymphocytes -are of several varieties and least is known of 

 them, in regard to their origin and function, on account of the difficulty 

 of study which they possess in common with other lymphoid tissues, 

 the movements of their cells which cannot be followed in life but must 

 be studied by successive stages in a series of different fixed sections. 



The myoid cells are evidently derived by a direct specialization of 

 some of the elements of the epithelial reticulum. These increase in size 

 and the cytoplasm shows a distinct fibrillation. The fibrils are parallel 



cil. 8. 



FIG. 279. A, portion of thymus tissue from an infant. Shows a ciliated space (cil.s.) (After 

 HAMMAR.) B, myoid cell from thymus of a small pickerel frog. X 850. 



and the groups form circular or whirlpool masses which center after a 

 fashion around the nucleus. These fibrils show a muscle-like segmen- 

 tation in some animals, as the frog (Fig. 279, B). The change may stop 

 here or may go on to the development of long strands of cytoplasm in 

 which the fibrils run parallel for some distance and show a strong seg- 

 mentation into anisotropic and isotropic segments that correspond to 

 those of muscle. 



Certain of the cells of the epithelial reticulum, situated always in 

 the medullary substance, become enlarged and grouped into concentri- 

 cally arranged masses which are known as "Hassall's bodies" from their 

 discoverer (Fig. 278). At first small and solid, these bodies increase 

 in size and the central portion degenerates and breaks down. The sev- 

 eral concentrically arranged and stratified peripheral layers show many 

 characteristics of a stratified epithelium, and it was considered by many 

 histologists that these hollow bodies were the remnants of the epithelium 



