470 JOHN BEARD, 



1) When, as shown in preceding pages, the first leucocytes arise 

 in the thymus-epithelium or placode, 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 would follow, 

 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 came from the thymus. 

 If a new outbreak of an infectious disease occur in a street at a 

 time, when in another part of the same thoroughfare the like illness 

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

 considered 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 inhabitants 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. From my own researches extending over 

 the period of the unfolding of the parts of the embryo, in Scyllium cani- 

 cula up to embryos of 33 mm, in Raja hatis up to young skate of 

 7 cm, there is no other organ laid down, which can possibly be re- 

 garded as a source of leucocytes. Attempts have been made by 

 Retterer and others to prove such a mode of origin of lymphoid 

 structures in the cases 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 embryologistand histologist, Philipp Stoehr ^). 

 All other supposed modes of development of leucocytes, except as emi- 

 grants from the thymus, or the direct descendents of such, have, 

 therefore, no basis of fact in their favour. 



3) The thymus alone is sufficient to account for all the leuco- 

 cytes of the body: and, since Schapfer (1894) demonstrated its exist- 

 ence in the lamprey, we know it to be an organ characteristic of all 

 true vertebrate animals. Any other source of leucocytes is 

 superfl uous. 



4) Except in the cases of organs, which are paired, or form 

 parts of a metameric segmentation, we do not seek for two organs 



1) For an account of this controversy see: J. Disse, Das reti- 

 culare Bindegewebe, in: Ergebn. Anat. Ent, V. 7, p. 7 — 28, 1897. 



