190 SPECIAL HISTOLOGY. 



not from a direct metamorphosis of the nuclei and cells in the 

 wall of the glandular lobule, but from the successive deposition 

 of an amorphous material around them ; and consequently that 

 they are analogous in their mode of formation to the corpuscula 

 amylacea of the brain, the prostatic concretions, &c. The 

 laminated portion consists of a substance certainly not of a 

 fatty nature which offers considerable resistance to alkalies, 

 approaching the colloid substance and the substance of the 

 prostatic concretions and corpuscula amylacea, being probably 

 formed from a change in the albumen in the glandular walls. 

 The situation of these concentric corpuscles is external to the 

 secretion of the thymus, and principally in the innermost part 

 of the glandular parietes, where the larger vessels occur. 1 ] 



§ 184. 



Development of the thymus. — According to Remak, the 

 thymus of the Chicken originates in the separation, by con- 

 striction, of the borders of the last two (third and fourth) bran- 

 chial fissures, which are lined by the intestinal epithelium ; and 

 at the period when the last three aortic arches become 

 detached from the walls of the pharynx, these follow them, and 

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

 them. 



In the earliest state observed in the Mammalia, e. g., in the 

 foetal Calf, l" long, according to Bischoff the gland represents 

 two delicate tracts of blastema, which descend from the larynx 

 as far as the thorax and appear to be connected above with 

 the thyroid body. Simon gives a similar description of the 

 thymus in the foetal Calf and Swine, § — 1±" long, except that 

 he makes no mention of any connexion with the thyroid body, 

 figuring the tract as a tube bounded by a delicate structure- 

 less membrane and filled with nuclei and a granular sub- 

 stance, which is further developed by becoming thicker and 

 longer, whilst at the same time it pushes out, at first, simple 

 and afterwards, more and more widely ramifying buds. Thus 



1 [Whatever may be the true nature of the Hassallian corpuscles, they do not 

 seem to be an essential element of the thymus or its secretion, as they are not found 

 in that body in Fishes (Skate, Ray, Sturgeon, Zeusfaber) ; although present in the 

 Reptilia. Leydig, 'Anat. histol. Untersuchung. u. Fische und Reptilien,' pp. 21 and 

 66.— Eds.] 



