THE DUCTLESS GLANDS 



309 



to follow, and have been the subject of much study and controversy. 

 The two most distinctly formulated views of its origin are, first, that it 

 is an epithelial invagination to which has been added a mesodermal 

 reticulum in which many leucocytes and other more specialized cells 

 have emigrated; and secondly, that it is a purely epithelial structure, 

 some of whose rapidly multiplying epithelial cells have formed a reticulum, 

 while others have become specialized into several kinds of cells, the larger 

 number of which closely resemble leucocytes. Many adherents of this 

 view approach a mean by saying that true leucocytes of mesodermal 

 origin do move in among the thymus cells, especially in the outer or 

 cortical part. They sometimes carry this idea to the extent of a third 

 theory, which claims that the cortex only of the lobules is mesodermal 

 in origin. 



Upon examining the thymus gland of a cat at about the time of birth, 

 we find the gland is composed of a fairly large number of solid, cellular 

 lobules which each ex- 

 hibit a rather weakly 

 defined cortical and a 

 medullary region. These 

 lobules are packed 

 closely together to form 

 an elongated mass of tis- 

 sue, and the medullary 

 portion of each, on the 

 inner side of the lobule, 

 is continued through the 

 cortex as a cord of tis- 

 sue which emerges to 

 unite with the similar 

 cords from other lobules 

 and thus form a central 

 connecting mass similar 

 to the medulla of each 

 lobe. Light coverings of connective tissue surround each lobule and 

 carry a heavy blood supply into its tissue. 



Under a high power (Fig. 278) the gland appears to be a reticulum 

 of connective-tissue cells in whose meshes there lie very many lymph 

 cells and other cells. As shown by Hammar, this reticulum originated 

 from a lumenless and ductless, invaginated mass of epithelial cells from 

 the region of the third embryonic gill cleft. This mass became reticular 

 by the formation of processes, from the cytoplasm of its cells, which re- 

 mained united with the processes from other epithelial cells. By the 

 lengthening of the connecting strands thus formed a fairly wide-meshed 

 reticulum is produced. 



FIG. 278. Small central portion of a lobe of the thymus 

 gland of a kitten. Has.b., Hassall's body; ret.c,, reticular 

 cells; th.lu., thymic leucocytes. X 800. 



