420 JOHN BEARD, 



the heart. In an embryo of 17 mm a single leucocyte was detected 

 in the thymus-placode of the first branchial cleft. Nowhere else in 

 this embryo were any other leucocytes encountered, and there were 

 none outside the placode within the mesoderm, as is invariably the 

 case in slightly older embryos. The remaining cells of the placode 

 were columnar. In another embryo of 18 mm three well-marked 

 leucocytes were detected within a single transverse section of the 

 thymus-placode, and there were also two cells in the act of taking on 

 leucocytic characters. Near this was a single leucocyte in the mesoderm. 



Since the end of 1899 other embryos have been studied, and 

 from these it may be gathered, that the youngest embryos, in which 

 leucocytes may be present, are some of about 14 mm. In R. hatis 

 No. 193, which is rather small (15 mm) for its characters, 130 so- 

 mites were counted, and all the gill-clefts were open. Here there 

 were a few leucocytes in the blood, but also some leucocytes and cells 

 becoming such in the anterior thymus-placodes. In another embryo, 

 No. 638, of the like period, but rather larger (16 mm) no leucocytes 

 were detected in either placodes or blood. In still another of the 

 same size and characters as the last, there were a few leucocytes in 

 the placodes and also in the blood. R. hatis No. 632 was about 16 mm 

 in length, in it there were no leucocytes in the blood, but some in 

 formation within the placodes. Two embryos of about 14 mm and of 

 like characters — among other things each possessing 104 somites 

 — are of interest. They are numbered Nos. 158 and 159. While 

 No. 159 had no fully formed leucocytes in the placodes or blood, in 

 No. 158 there were leucocytes in the placodes, one or two in the 

 mesoderm, and one was also noted in the blood near the placode. 

 In embryo No. 198 there were a few almost fully developed leuco- 

 cytes within the placode, but none were made out in the mesoderm 

 or blood. 



Summing up, wherever in the embryo leucocytes were absent in 

 the mesoderm and blood, there also none were in the placodes or they 

 were only in formation, and, conversely, in all the cases where leucocytes 

 were encountered in the former, there also without exception they 

 were met with in the latter. In some few instances the formation of 

 leucocytes could be made out in the placodes, before there were any 

 outside of these. 



As elsewhere already indicated ('00, p. 559—562), the period of 

 commencing histogenesis of the thymus-placodes and of the formation 

 of the first leucocytes in Raja hatis must be referred to embryos of 



