HISTOLOGY OF THE THYMUS 301 



dibiilae muscle. On the surface, one may see in the gross stellate 

 masses of black pigment. The surface is smooth and shows no 

 lobulation. After prolonged starvation, or in infected frogs, 

 the gland shrinks, has a somewhat translucent appearance and 

 may be difficult to recognize with the unaided eye. In old frogs, 

 the texture of the gland becomes firmer and the color a deeper 

 yellow. 



Microscopically, one finds a sharp distinction between cortex 

 and medulla, the relative proportions varying in different indi- 

 viduals. The cortex is composed principally of closely packed 

 small thymus cells, but often shows a subdivision into an outer 

 zone, in which the nuclei of the small cells are paler and show 

 a more distinct chromatin network; and an inner zone, in which 

 the nuclei are smaller, denser and more deeply stained. Mitoses 

 in variable number are present in both strata of the cortex, 

 but are probably more abundant in the periphery. 



The medulla is formed in large part by the myoid cells, which 

 in the frog's thymus are often a striking and conspicuous feature 

 of the histological picture. These cells have in general an oval 

 or circular outline which is quite sharply defined, although 

 band-like forms occur. With Heidenhain's iron hematoxylin or 

 with crystal violet after Benda fixation, they show a distinct 

 fibrillar structure. The fibrils most commonly have a circular 

 arrangement, and show an alignment of deeply staining rods or 

 granulae, giving the appearance of distinct cross striation. In 

 many cells of this type, however, there is no regular alignment 

 of rods or granules, which may be quite irregularly disposed 

 throughout the cell. Whether the granulae represent cross sec- 

 tions of short filaments, or true granulae, is often quite difficult 

 to decide. Many cells show in their peripheral portion, regularly 

 disposed striated fibrils, while in the central portion of the cell, 

 these appear to be broken up into dots and lines. Cells of this 

 type containing scattered clumps of deeply stained material, are 

 interpreted as degeneration forms. When the granules or rods 

 are at the same level in adjacent fibrils, the resemblance to the 

 striations of muscle fibers is a close one, and the aptness of the 

 term myoid is evident. 



