564 



The emigration of leucocytes from the thymus, as just described, 

 can be observed in other sections from the same embryo. Even if it 

 were really open to doubt as occurring in the present embryo, and 

 this is certainly not the case, the evidences for it, to be produced 

 from older embryos, are so overwhelming as to silence all objection. 

 In fine, there are cells in the act of becoming leucocytic and fully 

 developed leucocytes here in the thymus, here and there a leucocyte 

 is wandering out from the thymus - placode into the mesoderm, and 

 in the latter, in the immediate neighbourhood of the thymus, there 

 are also numerous leucocytes. These are evidently beginning their 

 amoeboid wanderings. In this embryo the leucocytes in the anterior 

 cardinal vein, and in the heart are fairly numerous. 



The mode, in which the epithelial cells become converted into 

 leucocytes, can be readily observed under high magnifications in em- 

 bryos of 17 to 22 mm, all well as in slightly older ones. Without 

 here describing the details at length, it may be stated, that the pro- 

 cess is exactly comparable to that, in which the original epithelial 

 cells of the brain or spinal cord become converted into ganglion-cells. 



As is the case in the latter, the original parent -cells of leuco- 

 cytes are epithelial cells. To study the conversion of these into leuco- 

 cytes it is needful to examine the parent - epithelium of the thymus 

 under a high lens such as a 2 mm apochromatic or a Vi2th oil- 

 immersion. 



The first change in such an epithelial cell appears to be one in 

 its cytoplasm. This becomes somewhat more refractive, and in favour- 

 able sections takes on a somewhat brownish tinge. 



At first no alteration is noticed in the nucleus, which is oval, as 

 in the other epithelial cells. 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 regard- 

 ing 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, while others of them pro- 

 ceed to wander out into the mesoblast and elsewhere. 



Those which remain seem to undergo division, for only in this 

 way can the numerous cell-nests^) of them and little groups of twos 

 and fours be explained. 



1) To be interpreted as the original "germ-centres" under the views 

 Flemming and his pupils. 



