404 JOHN BEARD, 



Originally, it had been intended to have written an account of 

 the work in greater detail. Thus, it was hoped, that time might 

 have been found for the completion of a set of observations upon the 

 degrees of development of skate-embryos at various periods. The 

 pressure of other research, more especially upon the germ-cells, has 

 indefinitely postponed the carrying-out of this. So that the division 

 of the chapters, finally adopted, has to some extent been based upon 

 the external features of embryos, and not in all cases has it been 

 found possible to control this by the record of all their internal 

 characters. 



Although little or nothing has been added to the notes and 

 drawings of the present work during the last two years, the WTiter 

 would still have kept back its final publication, had it appeared at 

 all likely, that the thread of research into the thymus would ever 

 again be taken up by himself. So far as the writer is concerned, 

 however, the work is complete, and there is now no point in the 

 history of the thymus of the skate, concerning which he would desire 

 further information from new research. At one time, some two years 

 ago, it did appear, that a much more thorough investigation of 

 Hassall's concentric corpuscles in the cavy would require to be made. 

 But a recent study of the preparations and facts, revealed during the 

 past two years' work upon the germ-cells, have rendered this, to my 

 mind, a task of supererogation. Possibly from the examination of 

 cavies rather younger than 31 days a little more information might 

 be obtained, but would it be such as to repay the labour? 



On the other hand, the facts, concerning the degeneration of 

 certain germ-cells with the formation of concentric capsules, and the 

 remarkable concentric corpuscles of the cavy itself — hitherto, so far 

 as I am aware, not described in the literature — demonstrate as 

 clearly as possible, that Hassall's corpuscles are only degenerative 

 products. Whether of leucocytes or of epithelial cells may be dis- 

 cussed upon another page. 



A few statistics concerning the embryos used for the following 

 description of observations may be of interest. The results are based 

 upon the study of upwards of 125 embryos, ranging from 6—70 mm. 

 This is, of course, only a portion of the writer's sectioned material of 

 Raja hatis. The figures of the thymus-elements and placodes are 

 from only 28 specimens, and those of the spiracular thymus from 

 but 5. Even with the restrictions of 28 embryos the number of figures 

 is rather large, and many of them may be described as duplicates. 



