594 SPECIAL HISTOLOGY. 



by the intestinal epithelium ; and at the period when the last three aortic 

 arches become detached from the walls of the loliarynx, these follow 

 them, and eventually lie as two elongated sacculi, on each side, between 

 them. 



In the earliest state observed in Mammalia, e. g., in the foetal Calf, 1 

 line long, according to Bischoff the gland represents two delicate tracts 

 of blastema^ which descend from the larynx as far as the thorax and ap- 

 pear to be connected above with the thyroid body. Simon gives a simi- 

 lar description of the thymus in the foetal Calf and Swine, l-l^- inches 

 long, except that he makes no mention of any connection with the thy- 

 roid body, figuring the tract as a tube bounded by a delicate structure- 

 less membrane and filled with nuclei and a granular substance, which is 

 further developed by becoming thicker and longer, whilst at the same 

 time it pushes out, at fi.rst, simple and afterwards, more and more widely 

 ramifying buds. Thus in foetal calves, 2^-3 inches long, wart-like and 

 spherical, in part even shortly pedunculated offsets already existed, 

 "svhich subsequently increased in number, becoming produced, at first into 

 two and four and afterwards, successively, into still more globular bodies, 

 until ultimately the lobules were completed. In this way the primitive 

 tube would be converted into the central cavity of the tliymus and eacb 

 offset of it, in course of time, into nn entire lobule of the organ. In 

 the human subject, as early as in the seventh week, I have seen the 

 thymus lobate at the lower end and single above. In an embryo ten 

 •weeks old, the upper extremity was a delicate walled tube, 0-04-0'06 of 

 a line in diameter, filled with polygonal cells ; the lower portion, 0-16 of 

 a line thick, presented several rounded outgrowths, in part isolated, in 

 part grouped, to the number of from two to five together. The thicker, 

 inferior portion of the gland, was entirely covered with further developed 

 lobules 0-08-0-1 of a line in size, of which, again, more simple gland- 

 granules, each with a structureless membrane and contained cells, w^ere 

 visible. In the twelfth week I found the thymus not much larger, but 

 the horns broader, and, like the rest of the organ, covered with lobules 

 0-12-0-24 of a line in size. From this, although the first stages have 

 not yet been observed in Man, there can be no doubt that the mode of 

 development is the same as that observed by Simon in other Mammalia. 



The later development of the thymus affords other interesting con- 

 ditions. In the embryo it continues to grow slowly from the third 

 month; in the sixth it extends as far as to the thyroid gland; and 

 from and after the seventh month begins to contain a whitish secretion. 

 After birth it is not stationary, as was formerly believed, but usually 

 continues to grow until the second year, and, indeed, at first very con- 

 siderably. After that period its growth ceases, though it still usually 

 remains for some time longer unchanged, until ultimately it becomes 

 atrophied, and finally disappears. The period at which these changes 



