565 



The emigrants are such, and not really immigrants. Until some 

 of them are formed within the epithelium, there is no source in the 

 body, whence they could be derived. Neither do they enter it as 

 epithelial cells or connective tissue-cells, or as "mesenchyma", for there 

 are no evidences whatever of this. In fine, in early stages all the 

 evidences go to prove the gradual conversion of the epithelial cells 

 of the thymusplacode into leucocytes. 



In his work on the thymus Stieda asks "how can adenoid tissue 

 arise from an epithelium?" The difficulty of understanding this may 

 have been a real one in 1881, when there were no observations 

 whatever, beyond Koelliker's discredited but correct ones, concerning 

 the origin of lymphoid cells. There is no longer any difficulty about 

 the matter, when many convincing preparations can be produced to 

 show it. 



Where Nature places no obstacle in the way of carrying out a 

 process, it is not for Man to invent one! 



The fact is, that in nature, as Koelliker stated in 1879, adenoid 

 tissue does arise from epithelial cells. 



In embryos of 22 mm and smaller the production of leucocytes 

 is not great, and their emigration from the thymus-placode can rarely 

 be observed. And, as we have seen, there are comparatively few leuco- 

 cytes within the latter. 



In embryos of 25 mm progress is being made. Leucocytes and 

 leucocytic cells are far more numerous within the epithelium, from 20 to 

 30 being met with in a single transverse section. Their emigration 

 is now a more conspicuous phenomenon. One may still meet with 

 single leucocytes, caught in the act of emigration, but this has begun 

 to give place to a slightly different process to be presently described. 



In an embryo of 29 mm the first thymus-placode has increased 

 greatly in size, owing to the proMferation of leucocytes within its 

 interior. In transverse section it bulges inwards from its attachment 

 as a great rounded mass. The majority of its original epithelial cells 

 are now leucocytic, and only comparatively few of them near its base 

 and along its outer margins, where it is continuous with the epithelium 

 above the cleft, retain their epithelial characters. Emigration of leuco- 

 cytes singly from the thymus has now become converted into a much 

 more startling process. When I now look at the sections showing 

 this — and there are hundreds of such sections revealing the same 

 thing in different embryos — my wonder is, not that the function of 

 the thymus is at last discovered, but that observers failed to detect 

 it several years earlier. 



