480 JOHN BEARD, The origin and histogenesis of the thymus in Raja batis. 



arranged epithelial cells, many in degeneration with chromatolysis. In 

 the centre of each a bright eosin-stained mass of highly degenerate 

 cells. In Figs. 58 and 59 entangled leucocytes, some in degeneration. 



Plate 10. 



Fig. 61. A portion only of the spiracular placode of a Raja hatis 

 embryo of about 25 mm. Like the ordinary thymus-placodes of embryos 

 of 18 — 20 mm it is still made up of one layer of epithelial cells, 

 among which a couple of leucocytes is seen. No. 443, I, 7, 20. 



Fig. 62. The right spiracular placode s.p of an embryo of 27 mm. 

 Abutting on it the sensory epithelium s. e of the hyomandibular branch 

 of the Vllth nerve. Gl.hm marks the position of the hyomandibular 

 ganglion. The spiracular thymus placode is now composed of more 

 than one layer of epithelial cells, and among them leucocytes are more 

 numerous. No. 629, II, 1, 14. 



Fig. 63. A portion only of the left spiracular thymus-placode of 

 an embryo of 28.25 mm. The stain here was iron-alum-haematoxylin. 

 The epithelium is comparatively simple and here and there are leuco- 

 cytes among its cells. No. 627, II, 3, 16. 



Fig. 64. The left spiracular thymus-element of a young skate of 

 7 cm. Unlike the other thymus-elements of this period it is still 

 attached to the branchial epithelium. In this instance it is a conical 

 structure of epithelium containing here and there leucocytes. Here there 

 are no trabeculae and no blood-capillaries. No. 255, VIII, 4, last 

 section. 



Fig. 65. The left spiracular thymus-element of an embryo of 

 54 mm. Here the whole element is a flattened plate bounded by two 

 layers of epithelial cells, between which are leucocytes and numerous 

 irregularly arranged epithelial cells. At v blood -capillaries. No. 245, 

 IX, 1, 6. 



Fig. 66. A contrast to Fig. 64 is afforded by this figure of the 

 left spiracular thymus-element of an embryo of circa 56 mm. The 

 whole structure here has a boundary of columnar epithelium to its 

 outer side and one of flattened epithelial cells to its inner. The interior 

 is occupied by leucocytes with trabeculae and blood-capillaries. No. 239, 

 IX, 4th section, (frontal series). 



Fig. 67. A section through the third right placode of an embryo 

 of 34 mm. From the same embryo as Figs. 13, 17, 48 and 51. To 

 show the emigration of leucocytes. 



Fig. 68. The ninth section of eighteen through the 1. right placode 

 of an embryo of 36 mm. Haematoxylin stain. No. 542, III. 3, 7. 



Fig. 69. The sixth section of eleven through the 1. right placode 

 of another embryo of 36 mm. No. 541, III, 4, last section but one. 



Krommiiimsche Uuchdruckorei (lieruiauu Fohle) in Jena. — 23G1 



