569 



their way into the thymus, blood-capillaries are developed pari 

 passu, as Koelliker observed in mammals. By this observation, which 

 would receive detailed mention elsewhere, it is proved that the wan- 

 dering of mesoblast-cells into the thymus (which, by the way, only 

 happens in Raja long after all its original epithelial cells have become 

 leucocytic), is concerned in the formation of a connective-tissue 

 meshwork and blood-capillaries, and not at all of leucocytes. 



The spleen is now indicated by a slight projection on the 

 mesentery. 



In an embryo of 54 mm each thymus-element is still attached 

 by a narrow stalk. There is a well-marked connective-tissue capsule 

 around each, and this here and there sends in a septum. Outside 

 this capsule the organ i. e. each element of it, is washed by a net- 

 work of blood-capillaries. These capillaries are formed in connection 

 with the vessel to the inner side of the thymus, the anterior cardinal 

 vein. There are still no capillaries within the thymus. Emigration 

 has become more difficult, but there are plenty of leucocytes in the 

 meshwork of the capsule, and these wandering cells now seem to make 

 their way through this to reach the blood-capillaries. The septa 

 within the thymus are not very numerous, but they penetrate it com- 

 pletely, cutting it up into little blocks. The external lobulation is 

 not so marked as in the embryo of 71 mm where it is very pro- 

 nounced. An embryo of 60 mm is in all respects quite similar to 

 that of 54. 



In the 71 mm embryo the thymus-elements are free from the 

 epithehum of each cleft. The capsule is better developed than in the 

 younger specimen, but it still seems to permit leucocytes to wander 

 out singly. Indeed, it may be suspected that this emigration of 

 leucocytes is never entirely prevented by the capsule, even in adult 

 specimens. 



Blood-capillaries, affording much more efficient opportunities for 

 the transport of leucocytes to all parts of the body, are now met 

 with within the thymus, and these run along the connective - tissue 

 septa into it. 



Summary of Results. 



All the more important factors in the development and histo- 

 genesis of the thymus of Raja have now been briefly recorded. The 

 narrative of these begins with embryos of 6 mm and ceases with young 

 skate of upwards of 7 centimetres. 



