The origin and histogenesis of the thymus in Kaja batis. 405 



Had it been attempted to avoid any and every duplication, and 

 thus to reduce the total number of figures, it is possible, that the 

 results might have obtained a doubtful reception. And, indeed, has 

 it yet fallen to the lot of any writer upon the thymus, to write the 

 truth and to be believed ? ! ! From the actual embryos, represented 

 by one or more figures in the plates, ten times as many drawings 

 could easily have been brought together. Leaving out of account the 

 spiracular thymus- elements, owing to the existence of five pairs of 

 placodes in every embryo, the drawing of but a single section through 

 each from every one of the 28 embryos would alone have yielded 

 280 figures. Considering, therefore, the extensive series of embryos 

 employed, and the number of the thymus-elements or placodes, the 

 figures given may, perhaps, not appear to be unduly numerous. And, 

 I hope, each and every figure — for all are accurate representations 

 of the actual sections — may not be without value. 



The views, hitherto held as to the morphological nature of the 

 thymus, have rested upon speculation, rather than upon actual re- 

 search. The facts, established by investigation, such as those due to 

 KÖLLIKER, Prenant, Oscar Schultze, and the writer, have been 

 systematically rejected in favour of the baseless conjectures of Stieda, 

 with the result, that the very opposites of real facts have been, and 

 unfortunately still are, believed. But now, with the possible exception 

 of Stteda himself, there is probably no-one, who has studied the 

 embryonic history of the thymus, and who is prepared to maintain 

 Stieda's hypotheses concerning it. 



The thymus was apparently discovered, and certainly so named, 

 by Galen. Notwithstanding the fact, that after then and down to 

 our own days it formed an object of research for many investigators, 

 including Hewson, Hunter, Valentin, J. F. Meckel, Tiedemann, 

 GooDSiR, Arnold, Bischoff, Simon, Remak, and others, nothing of 

 real value, concerning its functions and developmental history, was 

 established. But in 1879 Kölliker announced its mode of origin in 

 mammals from the epithelium of a gill-pouch, and the conversion of 

 its original epithelial cells into lymph-cells or leucocytes. This dis- 

 covery was not, however, destined to be accepted without cavil. 

 Against it were set the suppositions, unsupported by proof, of Stieda 

 and His, that the leucocytes, undoubtedly present in the thymus of 

 any late embryo or foetus, as well as in that of older animals, had 

 migrated thither from the exterior, possibly from the mesoblast. In 

 this conclusion they have been supported by the researches of Dohrn, 



