418 JOHN BEARD, 



animal there is only one form of leucocyte, whether be called by 

 this name, or be known as phagocyte, white blood-cell, lymph- 

 cell, etc. 



III. The Origin of tlie first Leucocytes. 



From the later portion of the text and from the figures of the 

 present memoir it might be concluded, that it was a superfluous task 

 to demonstrate in detail the formation of leucocytes by the thymus. 

 From the figures alone it is clear, that the thymus is an organ, whose 

 sole -function in Raja is the production of leucocytes. And since in 

 the embryology of the skate no other organ can be found possessing 

 the like function, it might logically be concluded, that the thymus was 

 the one and only source of the leucocytes of the body. And, indeed, 

 even a second organ of this kind, beyond the six pairs following the 

 branchiomery, would be a redundancy. No other source of their 

 formation is known, for an origin from "mesenchyme", or from meso- 

 derm cells of unknown parentage, or from structures other than the 

 thymus-epithelium, has never been made out to be in the least degree 

 probable. 



Although the task of proving the thymus to be the source of 

 the first leucocytes of the body is in this way one of supererogation, 

 and albeit a most difficult one, the writer has rightly or wrongly not 

 spared time or pains in testing the correctness of the conclusion. 

 Numerous preparations from embryos of 12 — 18 mm have been 

 worked over not fewer than seven times, and on each occasion with 

 the like result. 



To probe the matter to the bottom is not so easy as might ap- 

 pear. Difficulties arise from two circumstances. These are, that at 

 first the embryo contains at the best very little blood at all, and it 

 is difficult to retain even this within the vessels. Further, the first 

 formed blood-corpuscles are very like leucocytes, being rounded (Minot), 

 and staining in much the same way as the latter. Soon after the 

 first leucocytes appear, this possible source of error vanishes, in that 

 the red corpuscles become bigger, but for a long time they are more 

 rounded than oval. There is, however, one point, by which it is al- 

 ways fairly easy to distinguish the two sorts of cells. In the red 

 corpuscles the nucleus is central, in leucocytes, as so well shown by 

 M. Heidenhain, it is excentric. 



Parenthetically, it may be added with reference to the latter 

 point, that the figures show no centrosomes for the simple reason. 



