The origin and histogenesis of the thymus in Raja batis. 479 



section a nest of leucocytes with a large break, out of which many 

 are emigrating into the mesoderm. No. 209, IV, 2, 2. 



Fig. 49. The fifth section of ten through the 1. right placode of 

 an embryo of 24.5 mm. Some of the leucocytes of the placode are 

 forming nests, others are in emigration, and at one point there is a 

 small cavity, out of which perhaps two leucocytes have passed. No. 633, 

 II, 4, 22. 



Plate 9. 



Fig. 50. The seventh section of thirteen through the 1. right 

 placode of a Raja radiata embryo of circa 27 mm. Original epithelial 

 cells here are very restricted in number, and, what is very character- 

 istic of this species, the whole interior of the placode has been con- 

 verted into leucocytes. No. 242, II, 2, 25. 



Fig. 51. A contrast to the preceding is afforded by this figure 

 through the 2. right jdacode of a H. batis embryo of 34 mm. Here 

 the original epithelial cells are more numerous, and the leucocytes of 

 the placode, instead of forming one large mass, are broken up by 

 epithelial bridges into small groups or nests. No. 209, III, 6, 9. 



Figs. 52 and 53. Two consecutive frontal (horizontal) sections 

 through the 3. right placode of an embryo of 33 mm. In Fig. 52 

 especially there are nests of leucocytes, and others are engaged in 

 emigration. No. 214, last section of fifth slide and first section of sixth. 



Fig. 54. A section through the 1. left placode of an embryo of 

 29 mm. To show the restriction of epithelial cells to the base of the 

 placode, the conversion of the rest into leucocytes, and the emigration 

 of the latter en masse. N.B. A whole plate of similar figures was made 

 from a series of consecutive sections, but by the writer rejected for 

 publication as unnecessary. No. 210, V, 1, 9. 



Fig. 55. From a thymus-element of the left side of an embryo of 

 54 mm. The basal part of the structure is alone depicted. The element 

 projects as a pear-shaped or baloon-like mass from the branchial epi- 

 thelium. Over its base epithelium has now grown. In the element 

 itself all the cells are now leucocytes, and epithelial cells are entirely 

 absent. Connective tissue trabeculae penetrate it everywhere. No. 245, 

 XI, 1, 2. 



Fig. 56. A small portion of the 3. right placode of an embryo of 

 33 mm under high magnification (as lithographed 2/3 of 2250 diameters). 

 To show a leucocyte in the act of emigration from the placode. No. 214, 

 VI, 4th section. 



Fig. 57. A section through the 1. left placode of an embryo of 

 33 mm. Almost all the epithelial cells with the exception of those at 

 the base of the placode have been converted into leucocytes. The 

 figure exhibits emigration of leucocj'tes in crowds. No. 215, III, last 

 row, 12. 



Figs. 58 and 59. Show sections through Hassall's concentric cor- 

 puscles (concentric capsules) of a cavy-foetus of 31 days, and Fig. 60 

 of a similar body from one of 42 days. In all three concentrically 



