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arose in the mesoblast in the neighbourhood of the thymus. One 

 cannot help wondering whether or not he put the further question to 

 himself "Whence do they come"? At any rate he gave no correct 

 answer to this query, and the first reply to it is contained in the 

 present communication. 



It is Koelliker's great service to have shown that leucocytes 

 arise in the thymus from its original epithelial cells; to Gulland's 

 researches we owe the result that the first leucocytes are found in 

 the mesoblast in the neighbourhood of the thymus; and, finally, it has 

 fallen to my lot to show that the first leucocytes arise in 

 the thymus from its epithelial cells; and that thus, as 

 may presently be demonstrated, it must be regarded 

 as the parent-source of all the leucocytes of the body. 



The last link of the chain, binding the whole together, has 

 now at length been forged, and that this is so may be shown in an- 

 other way. 



Is there any other probable source of leucocytes in the verte- 

 brate body ? The answer to this question must now be a most decided 

 negative, and for the following reasons: 



1) When the first leucocytes arise in the thymus-epithelium there 

 are no leucocytes and no lymphoid structures of any sort in any other 

 part of the body. The first or parent-leucocytes by their wanderings 

 quickly infiltrate the blood and most other parts of the body. From 

 this it follows, that, if a lymphoid organ arise later elsewhere, it will 

 always be impossible to prove, that it did not take its origin from 

 some of the leucocytes, or their progeny, which originally cam« from 

 the thymus. If a new outbreak of an infectious disease occur in a 

 street at a time, when in another part of the street the same illness 

 has already manifested itself, the latter or its original source is con- 

 sidered sufficient to account for the fresh cases. The white races of 

 America are descended from ancestors, who emigrated from Europe, 

 and no one would dream of enunciating the idea that the white in- 

 habitants of, say, a western prairie village had arisen de novo in 

 loco. 



2) No other lymphoid organ is known, which in its developmental 

 history resembles the thymus. Attempts have been made by Retterer 

 and others to prove such a mode of origin of lymphoid structures in 

 the case of the tonsils, parts of the alimentary canal, and the Bursa 

 Fabricii of birds, but these have all one after the other been disproved 

 by an able embryologist and histologist, Philipp Stöhr. All other 

 supposed modes of development of leucocytes, except as emigrants 



