553 



attack of what at first seemed to be an entirely new problem, but 

 one which ultimately resolved itself into a solution of the thymus. 



The following results have been obtained by observation and by 

 it alone. In the course of a systematic investigation of the develop- 

 ment of Scy Ilium canicula, in aid of which the authorities of the 

 University of Edinburgh had given me a grant from the Earl of Moray 

 Research Fund, and, more particularly, whilst noting for subsequent 

 publication the periods of origin and degree of development of all the 

 systems of organs in a very extensive series of embryos, a little but 

 significant fact forced itself into notice. This was that for a relatively 

 long period of the development the blood contained only nucleated 

 coloured corpuscles. The circumstance was recalled that this had 

 originally been noted by Koelliker some years ago, and that it had 

 been commented upon by H. Ernst Ziegler, who, moreover, had 

 ventured the conjecture — a very safe one — that the condition per- 

 sisted until some lymphoid organ or other arose. 



The question was asked: "at what period of the development and 

 from what source or sources do the white corpuscles of the blood or 

 leucocytes arise?" It was obvious that, if the existence of a critical 

 period had a groundwork of fact, the leucocytes i) of the blood, as 

 forming integral parts of a vertebrate animal, ought then to be pre- 

 sent, or if not themselves evident as such, the foundation or found- 

 ations from which they arose ought to be in existence. 



In my fishes, in both Scy Hi um and Raja, it was very easily 

 made out that abundant leucocytes were present in the blood at, and 

 even before, the critical period. Working to earlier stages from this 

 point, and from early stages, where no leucocytes existed anywhere 

 in the embryos, the period was at length reached when leucocytes 

 first entered the scene. 



I had previously for both Scy Hi um and Raja made many 

 Lotes as to the degree of the development of the thymus at various 

 periods; intending one day with the aid of these, to make a new at- 

 tempt to find the solution of the thymus -problem. When the point, 

 at which leucocytes appeared, had approximately been fixed, a dilemma 

 was the first result. For the moment the observer had not the slight- 



when it is first beginning to resemble its progenitors. It then bears no 

 absolute likeness to them, but it is just beginning to look like them". 

 — J. Beard, The Birth-Period of T r i c h o s u r u s v u 1 p e c u 1 a. Zool. 

 Jahrb., Morph. Abt., Bd. 11, 1897, p. 87. 



1) With others I adopt the view that leucocytes, phagocytes, white 

 blood cells, and lymph-cells are identical structures. 



