436 JOHN BEARD, 



withiu the thymus are not very ouiiierous, l)ut they penetrate it com- 

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

 not so pronounced as in the next case, where it is very marked. An 

 embryo of 60 mm (No. 100) is in all respects like that of 54 mm. 



Raja hatis No. 255 measured 71 mm. The elements are now 

 freed from the epithelium of each cleft. The capsule is better de- 

 veloped than in the younger specimens, but it would still seem 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. The lobulation is marked, and the 

 connective tissue septa are well developed. Here and there obvious 

 blood-capillaries pass along the septa into the thymus. These, of course, 

 afford much more efficient opportunities for the transport of leucocytes 

 to all parts of the body. 



Beyond this point it is not proposed to carry the account of ob- 

 servations, although in the collection there are young fish up to those 

 of 19 cm, which have only quite recently left the egg- capsule. A 

 selection of these has been studied, but the thymus of No. 255 is 

 quite like that of a newly hatched skate. 



In these older specimens and in adult skate I have never come 

 across epithelial elements in the form of Hassall's concentric cor- 

 puscles. As elsewhere (1900, II, p. 570) already stated, it is neither 

 affirmed nor denied, that such bodies may obtain in old skate; but in 

 default of any means of identifying such an animal and determining 

 its age, the quest would be a hopeless one, and it may fitly be left 

 to those, who believe the degenerate structures known as Hassall's 

 concentric corpuscles to be of morphological, if not of physiological, 

 importance. 



In the foregoing account of observations the writer has, to the 

 best of his ability, endeavoured to confine himself to statements cap- 

 able of proof, and he believes, that none have been made without 

 at the same time the production of the evidences. This has been the 

 attitude towards the thymus and its problems uniformly adopted by 

 those, who with the writer maintain, that it produces leucocytes from 

 its original epithelial cells. The other view, to wit, that the leuco- 

 cytes of the thymus have wandered in from the exterior, and that 

 its original epithelial cells give rise to concentric corpuscles, has 

 from start to finish, as will be shown in another chapter, been 

 based solely on reiterated assertion without any real attempt at 

 proof. 



