570 



There are five thymus-elements or placodes on each side and all 

 of these persist, though the fifth remains small (Dohrn). 



Each thymus-placode arises as a specialised portion of the dorsal 

 epithelium of one of the five true branchial clefts, and it can be 

 identified as such, even when the cleft is merely in the pouch-condi- 

 tion. It follows, therefore, that the thymus of Raja is certainly of 

 hypoblastic origin, although for a period each placode comes to oc- 

 cupy an external or "epiblastic" position. 



As KoELLiKER first Stated, for a period of some duration the 

 embryonic blood is destitute of leucocytes ; and my researches have 

 revealed the fact, that this is so, until the first leucocytes are formed 

 within the thymus-placode and from its epithelial cells. As soon as 

 a leucocyte is formed in the early history of the thymus, this wan- 

 dering cell, true to its hereditary instincts, begins to emigrate from 

 the thymus, making its way into the mesoblast ; and sooner or later 

 many of them also get into the blood. The emigration of single 

 leucocytes only goes on as long as their production is not great. When 

 the thymus-elements set to work in earnest to form leucocytes, i. e. in 

 embryos of 28 mm and upwards, they (the leucocytes) emerge from 

 the thymus in crowds, thus causing larger or smaller "breaks" at 

 various places on its contour. 



In this way the whole body of the embryo, including its blood, 

 soon becomes richly infiltrated with leucocytes. This happens before 

 lymphoid structures are developed elsewhere within the body, and 

 demonstrably these first leucocytes of the blood and of the body have 

 taken their origin in one or other of the ten thymus-elements of 

 Raja. 



A capsule of connective tissue and blood-capillaries are formed 

 very late (43 — 54 mm embryos), and from this period the majority 

 of the leucocytes produced by the thymus probably make their way 

 directly into its capillaries. 



Hassall's concentric corpuscles have not been met with at any 

 period in the thymus of Raja. It is, of course, quite possible that 

 they may be found in old skate; but, as these animals possess no 

 certificate of birth, the search for such an example might not be suc- 

 cessful. 



General Remarks. 

 Before indicating the conclusions to be drawn from these results 

 it is fitting that the services of ray predecessors should receive notice. 

 The claims of other workers on the thymus would be set forth at length, 



