563 



The origin of the very first leucocytes of the body has now been 

 briefly sketched. The writer is fully conscious of the great importance 

 of a very close study and careful description of this part of the subject, 

 and in the complete memoir it would be treated of at much greater 

 length, and with more detail than given here. Moreover, it is ex- 

 pected that the material of the earliest stages may be still further 

 added to during the next few months (1898). 



In doing this it may also be of advantage to give fairly exhaustive 

 accounts of the degree of development of each embryo on the lines 

 of Keibel's "Normen-Tafeln". Only in this way is it possible to deter- 

 mine the actual value, as it may be termed, of each embryo. Size is 

 misleading, and even the enumeration of one or two attributes fails 

 to afford the accuracy yielded by the sum -total of the characters of 

 a certain embryo. 



In an embryo of about 22 mm the formation of leucocytes within 

 the thymus is proceeding apace. 



The thymus-placode is now nearly as thick again as in the em- 

 bryos previously described. And, whereas in the latter it had departed 

 but little from the original condition of a single layer of columnar 

 cells, this arrangement has been lost everywhere within it except at 

 the dorsal and ventral ends, as seen in transverse section. Without 

 possessing a very regular arrangement it is now composed of at least 

 three layers of cells. 



This epithelium of the thymus now contains many leucocytes and 

 cells in the act of becoming such. For instance, in two sections taken 

 through the median portions of the first thymus -element, there are 

 some half-a-dozen or more fully formed leucocytes and upwards of a 

 dozen cells, which are rapidly assuming leucocytic characters. 



Just beyond the inner boundary of the epithelium there are in 

 each section many leucocytes, lying in what may be termed the region 

 of the mesoblast. In the one section these are nine in number, in 

 the other thirty. How did these leucocytes come there? 



In both sections there are leucocytes so placed as to be in a 

 position to leave the thymus - placode by direct migration outwards,, 

 and in one of the two cases one leucocyte has been caught in the 

 act of emigrating from the epithelium. In this instance half the leuco- 

 cyte is within the latter, while the other half projects into the "terri- 

 tory" of the mesoderm. In such embryos there is absolutely no vestige 

 of a connective tissue capsule or other internal bounding membrane 

 in connection with the thymus. Such a structure, as will be seen anon, 

 only begins to be formed in embryos of double this size. 



36* 



