476 JOHN BEARD, 



leucocytes and many epithelial cells taking on the characters of such. 

 No. 189, II, 5, 10. 



Fig. 20. The first left placode shown in text-figure C, from an 

 embryo of 13 mm. The placode is made up of a single layer of high 

 columnar epithelial cells, and there are no leucocytes. The boundaries 

 of the placode are well-defined, and the remains c.m of the closing 

 membrane of the pouch are evident. No. 198, I, 6, 7. 



Fig. 21. A section through the 1. right pl'àcode of a, Raja radiata 

 embryo with 123 — 124 somites and six gill-pouches. The limits of the 

 placode are well marked, and in it are leucocytes and cells becoming 

 such. Close by are two leucocytes in the mesoderm. No. 195, I, 6, 2. 



Fig. 22. A section through the 3. right placode of an embryo of 

 25 mm. To the inner side are two leucocytes in the mesoderm while 

 others fully formed and forming lie within the placode. No. 201a, 

 III, 6, 5. 



Fig. 23. A section through the 1. right placode of the same 

 embryo. Here in correspondence with the more anterior situation there 

 are more leucocytes without the placode, and more such and of leuco- 

 cytic cells within it. (N.B. This preparation no longer exists, as owing 

 to the thickness of the cover-glass it was afterwards crushed.) No. 201a, 



III, 2, last section. 



Fig. 24. The median section of the 4. right placode of the same 

 embryo. The placode here still retains its original composition of one 

 layer of high columnar cells, but among them there are leucocytes and 

 epithelial cells in process of conversion into leucocytes, as well as such 

 in the act of emigration inwards and others in the mesoderm. No. 201a, 



IV, 1, 1. 



Fig. 25. From the same embryo as Fig. 18. The fifth section of 

 eleven through the 4. right placode. There are leucocytes in the meso- 

 derm and a few within the placode. No. 206, III, last row but one, 18. 



Fig. 26. From one of the newer series (1899) of embryos, No. 617. 

 The sixth section of ten through the 1. right placode. There are leuco- 

 cytes and leucocytic cells within the placode, and some of the former 

 in the act of emigrating. No. 617, I, last row but one, 16. 



Fig. 27. From the same embryo as Figs. 18 and 25. A section 

 through the 5. left placode. The latter is still merely composed of a 

 single columnar epithelium : within it there are leucocytes and the fore- 

 runners of such, one leucocyte is emerging, and many others lie in the 

 mesoderm to its inner side. No. 206, IV, 1, 16. 



Fig. 28. From the same embryo. The seventh section of twelve 

 through the 1. left placode. It is specially intended to show a large 

 nest of leucocytes, and single ones emerging from the placode. No. 206, 

 III, 4, 11. 



Fig. 29. From the same embryo. The seventh section of eleven 

 through the 2. left placode. Leucocytic cells and leucocytes are abundant 

 within the placode, and the latter are emerging in crowds from it, many 

 of them attaching themselves to the blood-vessel to its inner side. 

 No. 206, III, last row but four, 7. 



