424 JOHN BEARD, 



As in these instances, the original or parent-cells of leucocytes 

 are epithelial cells. To study the conversion of these into leucocytes 

 the parent-epithelium of the placode must be examined under such a 

 lens as a 2 mm apochromatic. The changes may be followed more 

 or less clearly in Figs. 22, 26, 27, and 34, etc. It had been intended, 

 to have drawn a set of figures under 1500 diameters or more to 

 illustrate this question, but the pressure of research in other directions 

 has hitherto prevented the carrying-out of this. The first change in 

 such a cell appears to be one in the cytoplasm. This becomes some- 

 what more refractive, and in favourable sections takes on a brownish 

 tinge. At first no alteration is noticeable in the nucleus, which is 

 oval, as in the other epithelial cells of the placode. Then the nucleus 

 becomes rounded, and, gradually, the whole cell acquires this shape. 

 With this and the more refractile nature of the cytoplasm the cell 

 has taken on the characters of a leucocyte. Another peculiarity, 

 already recorded by M. Heidenhain regarding leucocytes, is that the 

 nucleus comes to occupy an excentric position. 



Many of the earliest formed leucocytes apparently remain — at 

 any rate for a time — within the placode, whilst others of them 

 proceed to wander out into the mesoderm and elsewhere. Those, 

 which remain, would seem to divide often, for only in this way can 

 the numerous cell-nests of them, to be described at a later stage, 

 and the little groups of twos and fours be explained. These are 

 to be interpreted as the original "germ-centres" under the views 

 of Flemming and his pupils. 



The emigrants are such, and not really immigrants. The figures 

 ought to cany conviction of the truth of this. Until some of 

 them are formed within the placode, there is no source in the body, 

 whence they could be derived. Neither do they enter it as epithelial 

 cells, nor as connective tissue- cells, nor as "mesenchyme", for there 

 are no evidences whatever of this. In fine, in early stages all the 

 evidences go to prove the gradual conversion of the epithelial cells 

 of the thymus-placode into leucocytes. 



In his work on the thymus ('81, p. 24) Stieda asks "how can 

 adenoid tissue arise from an epithelium?" This is a matter for ob- 

 servation, and it may not simply be denied out of existence by a 

 mark of interrogation! The difficulty may have been a real one in 

 1881, when there were no observations whatever — even after the 

 publication of Stieda's own researches — beyond Kölliker's discredited 

 but correct ones, concerning the origin of lymphoid cells. As will 



