408 JOHN BEARD, 



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

 period in the development [in the sense applied to this conception by 

 the writer i)] had a groundwork of fact, the leucocytes ^) of the blood, 

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

 present; or, if not themselves evident as such, the foundation or 

 foundations, from which they arose, ought to be iu existence. 



In my fishes, in both Scyllium and Baja^ it was easily made out, 

 that abundant leucocytes were present in the blood at, and even long 

 before, the critical period, when the embryo first asserts its individuality. 

 Working to earlier phases from this point, and also from such early 

 stages, where no leucocytes existed anywhere in the embryo, the 

 period was at length reached, when leucocytes first entered the 

 scene. 



For both Scyllium and Baja many notes had previously been 

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

 periods; with the intention of their being used one day in a new 

 attempt 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 

 slightest idea whence they came. The writer could accept neither 

 Gulland's condensation-theory of the origin of leucocytes from con- 

 nective tissue cells, nor Ziegler's suggestion of their genetic con- 

 nection with the remains of the "mesenchyme". Kölliker's results 

 on the thymus, proving that it produced leucocytes, were remembered, 

 as was also my own confirmation of them. The comparatively early 

 phase ^), at which leucocytes appeared, did not lend colour to the idea, 

 that in the skate and dog-fish the thymus could have much to do 

 with the matter. For it was recalled, that in such embryos the 

 thymus-elements were then nothing more than small pieces of modified 

 epithelium. Although the thymus was not a priori a very promising 



1) Beard, J., Certain problems of Vertebrate embryology, Jena, 

 Gustav Fischer, 1896, p. 60. Vide also: Beard, J., The birth-period 

 of Trichosurus vulpecula, in: Zool. Jahrb., V. 11, Anat., 1897, p. 87. 



2) With others and for reasons to be given at a later stage I 

 adopt the view, that leucocytes, phagocytes, white blood -cells, and 

 lymph-cells are identical structures, and I cannot admit the morpho- 

 logical existence of more than one kind of leucocyte. 



3) Early only as regarded from the point of view of my own 

 work, for this often extends to periods, not usually dealt with in 

 embryological investigation. 



