568 



that in an embryo of R. batis of 71 mm all the foundations of all 

 the organs are laid down, that the embryo is rapidly making for the 

 adult form, and that, in fact, it is at this period to all intents and 

 purposes for the first time a young skate. 



In this species of fish, Raja batis, the thymus is not merely 

 laid down by the time this "critical period" is reached, but it has 

 undergone a very considerable degree of histological differentiation. 



As the main differences, seen in egg-capsule specimens of upwards 

 of 71 mm, concern the size of the thymus, and the complexity of its 

 network of connective tissue and blood capillaries, it will be obvious 

 that little new information of importance is to be got from their study. 

 The adult thymus of R. batis is in structure quite like that of a 

 newly-hatched skate. 



And — an important and significant fact — Hassall's concentric 

 corpuscles have never met my eyes either in embryos, young speci- 

 mens, or in adults of Raja. These are only known in fishes from 

 an observation of Maurer's. The single figure given does not, how- 

 ever, convince one that the structure depicted is really a Hassall's- 

 body. 



Returning to our starting point in describing the later history of 

 the thymus, this latter may be told in the brief form afforded by my 

 notes. 



In an embryo of 33 mm (greatest length) epithelial cells are now 

 restricted to the basal portion of each thymus-element. The emigra- 

 tion of leucocytes is still actively going on, there are no blood-vessels 

 within it, and it is destitute of a connective-tissue capsule. There is 

 as yet no trace of the spleen. This condition of affairs persists 

 until the embryro is 42 mm in length. 



In an embryo of 43 mm there are still no blood-vessels within 

 the thymus. A capsule of connective tissue seems to be in course of 

 formation, and it was noted that an epithelial covering was growing, 

 as over a wound, over the outer end of the base, i. e. of that part 

 next the gill-cleft. In an embryo of 45 mm this epithelial membrane 

 is complete. Within the actual thymus there are practically no longer 

 any epithelial cells left at the base of each element. Each element 

 presents marked indications of lobulation, and between the lobules 

 connective tissue is making its way into the thymus. A capsule is 

 evidently also forming around the organ. Whilst there are no signs 

 of blood within it, one or two capillaries lie around it. 



It may be remarked here, that in Sc y Hi urn and Pristiurus 

 one can easily make out that, as the connective-tissue strands force 



