4-00 DEVELOPMENT OF THE THYMUS. [SECT. 1 83. 



which I will call, with EcJcer, the concentric bodies of the thymus. They 

 appear in very various forms, which may, I think, be conveniently reduced to 

 two. The first form consists of simple bodies, of o - oo6'" to o - oi'" in size, 

 whose envelope is thick and concentrically striated, while their contents, 

 ordinarily granular, have sometimes the appearance of a nucleus, sometimes 

 of a cell. Secondly, we find compound bodies, measuring up to 0*04'", or even 

 o-o8'", which consist of several simple bodies, surrounded by a common 

 lamellated envelope. These structures, which were first made mention of by 

 Hassall and Virchow, and which were further followed up by Echer and Bruck, 

 appear to me to arise, not by direct metamorphoses of the nuclei and cells in 

 the walls of the lobules, but by the successive deposition of an amorphous 

 substance around them, and thus to be analogous in their mode of formation 

 to prostatic calculi. The lamellated part of these bodies consists of a sub- 

 stance which affords considerable resistance to alkalies, and is certainly not 

 of a fatty nature. Its affinities are rather with colloid substance, and the 

 substance of prostatic calculi, and it probably results from an alteration of 

 the albumen in the gland-tissue. In certain cases, the laminated substance 

 appears to consist of flattened cells, so that the whole bears a resemblance 

 to the laminated epidermoid growths of pathologists. The seat of these 

 concentric bodies is in the secretion of the thymus, but principally in 

 the innermost parts of the walls of the gland, in the situation of the larger 

 vessels. 



§ 183. Development of the Thymus. — According to Memah, the 

 thymus of the chick arises from the borders of the two last (third 

 and fourth) branchial fissures, and it becomes marked off from the 

 mucous surface lining these edges ; then, following the three last 

 aortic arches at the period when they detach themselves from the 

 walls of the pharynx, the thymus comes to lie between these, as 

 two elongated sacs. 



In the earliest condition seen in mammalia (in a .calf embryo, 

 1 inch long), the gland, according to Bischoff, consists of two deli- 

 cate streaks of blastema, which extend downwards from the larynx 

 towards the chest, and appear to be connected superiorly with the 

 thyroid. Simon gives a similar description of the thymus in the 

 embryoes ($ to 1^ inch long) of the calf and pig, only he mentions 

 nothing of a connection with the thyroid, and describes the central 

 thread of the gland as a tube limited by a delicate structureless 

 membrane, and filled with nuclei and a granular material. This 

 tube, he says, becoming thicker and longer, developes itself further 

 by sending out processes, which are at first simple, and afterwards 

 become considerably ramified. Thus, in the embryo of the calf, 

 ^ to 3 inches long, Simon found processes springing from the 

 thymus, which were rounded and papilliform, or even furnished 

 with short stalks. These processes increased in number by 

 branching out in twos and fours, until at last a number of 



