The origin and histogenesis of the thymus in Raja batis. 475 



Fig. 6. A section through the 1. right placode of an embryo of 

 circa 17 mm, 133 somites, showing one leucocyte in the epithelium. 

 No. 343, II, 1, 2. 



Fig 7. A section through the centre of the 1. left placode, show- 

 ing numerous leucocytes in the latter and cells becoming such, also the 

 emigration of leucocytes into the mesoderm. The topographical rela- 

 tionships of the placode of this figure are given in text-figure F. 

 No. 202, III, 2, 4th bottom. 



Fig. 8. A section through the 1. right placode of an embryo of 

 more than 22 mm, showing leucocytes within and to the inner side of 

 the placode, No. 192, III, 4, 11. 



Figs. 9 and 10. Two sections from an embryo of rather more 

 than 20 mm. Fig. 9 depicts the median section through the 1. placode 

 of the right side. Fig. 10 through the corresponding left one. In both 

 there are leucocytes and cells becoming such. Fig. 9, No. 190, III, 1, 

 3 bottom. Fig. 10, No. 190, II, 1, 4, 



Fig. 11. The median section of the 1. left placode from an embr3'o 

 of 25 mm. Within the placode there are cells becoming leucocytes, 

 leucoc^'tes themselves, and in addition there are a few leucocytes in the 

 mesoderm. No. 202, IV, 2, 12. 



Fig. 12. From the same embryo, but the median section ol the 

 1. right phxcode. The details are as in Fig. 11. No. 2(l2. IV, 2, G. 



Fig. 13. A small portion of the inner border of a section through 

 the 2. right placode of an embryo of 34 mm. Compare with Figs. 17 

 and 07 from the same embryo. The figure shows under high magni- 

 fication the nature of a "break'' close to the blood-vessel, and leuco- 

 cytes emerging in crowds. No. 209, III, 6, 9. 



Fig. 1 4. A frontal (horizontal) section through the 1. right placode 

 of an embryo of 25 mm, showing some leucocytes within the placode 

 and others emerging therefrom. No. 202a, V, 10th section. 



Fig. 15. The median section of the 2. right placode of an embryo 

 of 25 mm, showing the formation of leucocytes within the placode, 

 and a crowd of such emerging from the latter. No. 201a, III, 4, 12. 



Fig. 16. A portion of a horizontal section passing througli the 

 1. left placode of an embryo of 25 mm. Leucocytes and cells becom- 

 ing such within the placode, and a few of the former outside it. No. 203, 

 VIII, 5th section. 



Fig. 17. From the same embryo as Fig. 13. The median section 

 of the 1. right placode. Within the latter epithelial cells, leucocytes, 

 and cells becoming such. At various points leucocytes are emigrating 

 from the placode. The topographical relationships of this section are 

 shown in text-figure F. No. 209, II [, 4, 9. 



Fig. IS. A section, the fourth of eight, through the 3. left 

 placode of an embryo of 28 mm. No. 20(5, III, last row but two, 14. 



Plate (i. 

 Fig 19. 'J^he median section of the 1. left placode, shown in text- 

 figure E, from an embryo of circa 20 — 21 mm. There are one or two 



