458 JOHN BEARD, 



marise the observations here would merely be to repeat once more 

 the substance of these two papers. 



In a certain sense there is little previous history to be dealt with 

 here. For the present writing is more an embryological account of 

 the first leucocytes of the vertebrate body than a memoir upon the 

 development of the thymus. That it has also turned out to be the 

 latter is due to the circumstance, that the thymus happens to be that 

 source. In this way it has come about, that by a flank attack the 

 solution of the problem of the function of the thymus was also found. 

 Previous attempts at this obtained at the best only partial success, 

 because of their frontal nature^). The mistake hitherto, made by embryo- 

 logical observers with reference to the thymus, has been, that the 

 organ and its origin have been exclusively studied, instead of the 

 history of its products, the leucocytes. 



In the literature of embryology there is nothing concerning the 

 origin of the first leucocytes of the body. On the other hand, there 

 exists a not inconsiderable amount of literature treating of the de- 

 velopment and histogenesis of the thymus. The true history of the 

 histogenesis of the thymus — and there is no sadder one in the 

 archives of modern embryology — goes back to Kölliker's obser- 

 vations, published in 1879. The history of research into the thymus, 

 or rather speculation as to its nature, does not then commence, for 

 long ago Hewson thought it to be a source of red blood-corpuscles, 

 and GooDSiR suggested, that with certain other organs it formed the 

 remains of the "blastema", from which the embryo arose. And so on. 

 There would be no profit in reciting these ideas and speculations. 

 KÖLLIKER (1879) stated, that the thymus^of the rabbit arose from (the 

 wall of) a gill-cleft, and that its original epithelial cells became con- 

 verted into leucocytes. 



While confirming the origin, ascribed to it by Kölliker, and 

 this was soon afterwards still more firmly established by Gustav 

 Born (1883), Stieda (1881) in other points made statements directly 

 contradictory to the latter of Kölliker's finds. With the long inter- 

 val between 1881 and 1902 it might not have been necessary to have 

 regarded Stieda's work as other than a chapter of the ancient history 

 of the thymus, had Stieda not quite recently used what he now 



Anz., V. 18, p. 359 — 303, 1900, and The source of leucocytes and the 

 true function of the Thymus, ibid. V. 18, p. 550—573, 1900. 



1) This comparison of frontal and flank attacks in research, with 

 their consequences, is originally due to Ranvier. 



