THYMUS GLAND. 



1091 



pedicles (figs. 725. and 726.). This budding 



Fig. 725. 



Fig. 726. 



Second stage of deeelopement of primary tube forming 

 follicles. (After Simon.) 



out of the primary tube does not occur simul- 

 taneously at every part, or uniformly, but 

 chiefly at those situations which are ulti- 

 mately to attain the largest size ; thus in the 

 foetal calf we find tolerably v/ell developed 

 bulgings of the primary tube opposite the 

 angle of the jaw, the upper part of the trachea, 

 and the pericardium, while the intervening 



portions have smooth and undulated margins ; 

 and it is just the parts of the gland corre- 

 sponding to the above points, which ulti- 

 mately attain the greatest magnitude. The 

 third stage of developement consists in the 

 ramification of the follicles which have budded 

 out from the central cavity; they do not 

 usually elongate much, before they throw off 

 fresh offsets, and these are completely sessile, 

 so that they have the appearance of vesicles 

 or imperfect spheres grouped together : the 

 mode in which the primary offset divides is 

 either dichotomous or quaternary (fgs. 727. 

 and 728.), probably also often with some 

 degree of irregularity and inequality in the 

 size of contiguous offsets. By the extension 



Figs. 727, 728. 



Third stage of developement. 



Ramification of follicles by dichotomous and 

 quaternary division. (After Simon.') 



of this follicular growth to all parts of the 

 primary tube, and by successive lateral rami- 

 fications, occurring, as we have seen, to a 

 greater extent in some parts than in others, the 

 gland attains its mature size and complex 

 structure. In this state however it consists, 

 in very great measure, of vesicular cavities, 

 which cluster around and completely obscure 

 the primary tube from which they have origi- 

 nated ; yet tin's primary tube or reservoir does 

 exist, and is capable, as we have seen, of 

 being demonstrated, so that the term which 

 Mr. Simon has proposed, as expressing the 

 type of the mature structure, viz. tubulo- 

 vescicular, is sufficiently correct (fig. 729.). 

 Two varieties have been observed in the second 

 stage of the process; one is, that "the tube 

 sometimes bulges uniformly in its whole cir- 

 cumference for some extent, forming a very 

 distinct ampulla;" the other, " that in parts 

 where there are yet no bulgings, it is some- 

 times flexuous, or even contorted." 



The observations now detailed respecting 

 the progressive developement of the thymus, 

 are so important in the elucidation of its 

 structure, that I thought it very desirable to 

 repeat them, if possible, and confirm their ac- 

 curacy by independent testimony. 1 have 

 not, however, been able to procure a foetus at 

 a sufficiently early period to discover the 

 primary tube of the thymus, with its smooth 



4- A 2 



