THE THYMUS GLAND III 



clefts. It proliferates into the pleural space until it reaches the 

 heart, at the same time becoming gradually separated from 

 the branchial clefts. This sac-like primordial beginning of the 

 thymus is entirely epithelial in structure ; the lumen is very small 

 and the cortex is composed of several layers of epithelial cells. 

 Branch-like processes are early thrown out from the lower edge, 

 the organ in man assuming a lobular structure at the second or 

 third month of fcetal life. Connective tissue and blood-vessels 

 now begin to form and the communication between the central 

 lumen and the pharyngeal cleft closes up. At birth, the two 

 halves are grown together in the median line, and the thymus 

 lies between the sternum, the pericardium and the great vessels, 

 with the upper end extending upwards into the region of the 

 neck. 



The complete thymus is a greyish-red organ composed of 

 lobes and lobules, the lobules being surrounded by a delicate 

 sheath of connective tissue. The change which, in the course of 

 development, takes place in the length and breadth of the organ, 

 is accompanied by changes in its histological appearance, and 

 the original epithelial organ develops into a structure resembling 

 the lymph glands. 



There are two theories as to the manner in which this 

 histological change takes place. According to the one, the trans- 

 formation theory, the epithelial cells divide and subdivide, be- 

 coming smaller and smaller so as to resemble leucocytes. Accord- 

 ing to the other, the theory of pseudorrorphosis, the epithelial 

 beginning of the primordial thymus is, with the exception of a 

 small portion, entirely replaced by elements from the central 

 germinal layer. From investigations carried out with a large 

 number of animals of different species, Hammar concludes that 

 the reticulum the supporting and stellate cells, believed to be 

 derived from the central germinal layer is in reality also of 

 epithelial origin. He regards the reticulum as the most important 

 element of the thymus parenchyma. 



Development is accompanied by changes in the epithelial 

 thymus, both regenerative and degenerative in nature. In the 

 human embryo, signs of involution appear in the course of the 

 fourth month, the flat, horny epithelial cells becoming transformed 

 into the concentrically disposed layers of Hassal's corpuscles. At 

 the same time, leucocytes pass from the blood-vessels for the pur- 

 pose of absorbing the broken-down medullary masses. According 

 to Stoehr, the earlier view of the thymus as a lymphoid organ is 

 no longer tenable. And he also points out that the so-called 

 lymphocytes, upon the existence of which the analogy of the 

 thymus with the lymph glands rests, are of epithelial origin and 

 are epithelial in their permanent characteristics. 



In many animals, the complete thymus is apparently entirely 

 composed of lymphoid tissue, the epithelial cells having almost 



