468 JOHN BEARD, 



Its absence in the earlier ones is, of course, explicable, for not until 

 1900 was the existence of such a structure certain. From the facts 

 recorded here concerning the spiracular thymus it appears clear, that, 

 unless the whole history of the dorsal epithelium of any particular 

 pouch have been followed, the possible presence of a thymus-placode 

 in connection with it at some period of the early development cannot 

 be excluded. Where such a placode exists, even for a time, it may 

 give rise to more or fewer leucocytes, and then disappear. 



The facts, then, that the thymus-placodes must originally have 

 existed in connection with all the pouches, including the spiracle, this 

 condition still persisting in the skate, and that in some cases an ac- 

 cessory thymus-element has been described, place these comparative 

 tables in a different light. 



For, if the existence of a thymus-placode obtain in connection 

 with the dorsal wall of every gill-pouch, even though as a rule cer- 

 tain of these placodes disappear, they may function in a feeble 

 fashion before doing so, and occasionally one such may possibly persist- 



The foregoing account, therefore, has been limited to those re- 

 searches, which treat of the histogenesis of the thymus. The like 

 procedure has been adopted in the appended list of literature, although 

 from the lists given by Stieda, Watney, Prenant, Kohn, Livini, 

 GuLLAND, Maurer, and by Wiedersheim in his "Vergleichende Ana- 

 tomie" it would be an easy matter to bring together a very complete 

 account of the literature of the embryology of the thymus. 



No reference is made to the literature of the epithelial bodies 

 of Maurer and Kohn, for these do not exist in the skate, indeed, 

 they are probably absent in all fishes. Maurer's suggestion, that 

 Froriep's "Kiemenspaltenorgane" of Selachians may be the represent- 

 atives of such epithelial bodies, is not worthy of serious consider- 

 ation, and is but another example of the disastrous efifects of Froriep's 

 erroneous conclusions, which have been fully dealt with in a preced- 

 ing section. 



Lastly, it has not been thought necessary to enter into any ac- 

 count of the controversy between Retterer and Stoehr as to the 

 supposed various sources of leucocytes, advanced by the former. A 

 review of these questions from J. Disse's pen will be found in the 

 "Ergebnisse der Anatomie, etc.", V. 7. 



XIII. The Thymus as the Source of Leucocytes. 



The circumstance, that our supposed knowledge of the function 

 and histogenesis of the thymus has largely been based in error, 



