The origin aud histogenesis of the thymus in Raja batis. 419 



that the stain employed — usually picrocarraine — does not bring 

 these out. There are also in my possession many other preparations 

 of early thymus-placodes, stained with iron-alum-haematoxylin, in which 

 the centrosomes are quite prominent features. 



In my former publication ('00, p. 559) it was written, that the 

 first formation of leucocytes "begins as a rule in embryos of about 

 17 mm". That statement was made on the result of three separate 

 inquiries. Since then the preparations have been worked over other 

 four times, and there is no reason for altering the previous conclusion. 

 In one point only may a correction be made. It was added, that in 

 one embryo (No. 164) of 10 mm what appeared to be an undoubted 

 leucocyte was detected in a section of a thymus-placode. This ap- 

 peared to be so under the Via^^i oil-immersion, but when the fine 

 2 mm apochromatic lens of Zeiss was afterwards obtained, examina- 

 tion under it did not confirm the previous diagnosis, my note of this 

 reading "more than doubtful". 



In general terms it may be stated, that the two anterior placodes 

 of each side remain entirely epithelial, until the embryo is about 

 17 mm in 'length. Regarding the other three placodes nothing need 

 be added, for, on the one hand, but little attention has been paid to 

 them in the early periods, and, on the other, they undoubtedly, as 

 is natural, lag behind those of the first two clefts, and are still quite 

 simple epithehal plates in embryos of more than 20 mm in length. 



In embryos below 17 mm there is as a rule an entire absence 

 of leucocytes anywhere else in the body, and other than the thymus 

 no lymphoid structures whatsoever are present. In the course of the 

 investigation as first carried out "the examination of about 20 em- 

 bryos of 10 — 16 mm failed to reveal any leucocytes whatsoever 

 in the heart, or blood-vessels, or mesoblast, or in the thymus-pla- 

 codes". 



In one embryo of about 20 mm repeated examination of the thymus- 

 placode with the ^/i2th inch oil- immersion failed to reveal any fully 

 formed leucocytes within it, although some of the epithelial cells of 

 the structure were taking on what may be termed leucocytic charac- 

 ters. In this embryo there were few leucocytes in the blood and 

 heart. The explanation of the absence of fully formed leucocytes in 

 the thymus-placode here is, that the few leucocytes formed had already 

 wandered out. In another embryo of about the same size there appear 

 to be one or two leucocytes within the thymus-placode, and there are 

 also one or two outside of it in the mesoderm, as well as a few in 



27* 



