416 JOHN BEARD, 



DE Meuron's (1886) researches upon Elasmobranch fishes do not go 

 back to a sufficiently early period of the development to afiord any 

 information of the first origin of the thymus here. 



II. The Morphological Nature of Leucocytes. 



The object of the following description is to prove, not merely 

 that the thymus produces leucocytes — that point was established 

 long ago by Kölliker (1879) — but that it must be regarded as the 

 source of the first leucocytes, and, therefore, as the ultimate 

 birthplace of all the leucocytes. It need hardly be stated, that these 

 conclusions are directly contradictory to the teachings of pathologists, 

 and even of histologists. The discussion as to the existence of several 

 categories of leucocytes, at any rate in the higher animals, has been 

 a great one, especially among pathologists. It cannot fall within the 

 scope of the present writing to review the literature, or even the 

 arguments, on which various kinds of leucocytes have been re- 

 cognised. 



Undoubtedly, much of the supposed evidence has been furnished by 

 the reactions of leucocytes under various conditions (disease) towards 

 diverse stains, and morphologically distinct sorts of leucocytes have 

 been distinguished from phenomena noticed in disease. But there 

 have not been wanting among histologists and pathologists some, who 

 have rejected the idea of the necessary existence of more than one 

 kind of leucocyte. This was done by Gulland (1891, 1) in a very 

 fine piece of research, and the like standpoint has quite recently been 

 taken up by Friedrich Hesse (1902). Gulland's memoir contains a 

 long list of literature, and to this and the memoir itself the reader 

 may be referred for fuller information. The object of his work is to 

 prove, that the various forms of leucocytes, recognised by pathologists, 

 and to some extent by histologists, are phases of a definite life-cycle 

 of an organism, comparable to a Protozoon. Rewrites: "A leucocyte 

 is a unicellular organism, which, in the midst of the vertebrate tis- 

 sues, retains the character and habits of a Protozoon" (1. c. p. 113), 

 and further on: "the different kinds of leucocytes are all, in reality, 

 mere varieties of one ground form, or, to speak more exactly, are 

 stages in the life-history of a unicellular organism." 



Naturally, this conclusion cannot find acceptance on the part of 

 a pathologist, who believes he has witnessed the actual birth of one 



