The origin and histogenesis of the thymus in Raja batis. 477 



Fig. 30. From the same embryo. The median section through the 

 3. left placode. There are nests of leucocytes within the epithelium, 

 two of which are breaking into the mesoderm with emigration of their 

 leucocytes. No. 206, III, last row but two, 14, 



Plate 7. 



Figs. 31 — 36 are all from various placodes of the like embryo, 

 No. 443. It was not measured, but its size was probably 25 — 27 mm. 



Fig. 31. The fifth section of twelve through the 1. right placode. 

 The chief peculiarity of this (and of the others up to and including 

 Fig. 36) is the sudden conversion of whole masses of the original epi- 

 thelial cells into leucocytes, thus forming more or less well-marked 

 nests, usually sharply defined from the rest cf the original epithelium. 

 No. 443, I, 9, 11. 



Fig. 32. The eighth section through the 1. right placode. Here 

 the original epithelium is almost entirely confined to the dorsal and 

 ventral ends of the placode. The rest is almost all made up of leuco- 

 cytes. No. 443, I, 9, 14. 



Fig. 33. The eighth section of fourteen through the 2. right 

 placode. As in the preceding the epithelium is mainly confined to the 

 ends. Numerous leucocytes are in emigration from the placode. No. 443, 

 I, 10, 17. 



Fig. 34. A section through the 4. right placode under higher 

 magnification showing nests of leucocytes, single leucocytes, and cells 

 becoming such within the placode. The original epithelium is best 

 marked at the two ends. No. 443, II, 1, 18. 



Figs. 35a and b. Neighbouring sections through the last (5.) right 

 placode. As usually the case, in this placode the epithelial character 

 has been longer retained, but apart from cells becoming leucocytes, and 

 in addition to single ones, there is in Fig. 35b a nest of such with 

 a break into the mesoderm. No. 443. II, 2, 6 and 8. 



Fig. 36. The seventh section through the 2. right placode. As in 

 Fig. 34, there is here a large nest of leucocytes within the placode, 

 and this has used up in its formation practically all the epithelial cells 

 of this region, thus limiting the original epithelial cells to the two 

 ends. No. 443, I, 10, 15. 



Figs. 37 — 39 are from frontal (horizontal) sections of two embryos 

 of about 25 mm. 



Fig. 37. A section through the 2. right placode, showing epithelial 

 cells, such in process of conversion into leucocytes, and fully formed 

 leucocytes. Of the latter many are in emigration. No. 202a, VI, third 

 section. 



Fig. 38. Through the 4. left placode of the same section. Here 

 the leucocytes are fewer, but many epithelial cells are being converted 

 into such, and a few leucocytes are in emigration. No. 202a, VI, 

 third section. 



Fig. 39. A section through the 3. right placode. This is very like 

 the preceding. In addition it shows the boundaries of the placode and 



