442 JOHN BEARD, 



not be repeated here, aud the reader may be referred for them to 

 my original communication upon the spiracular thymus of Raja, cited 

 above. 



From the observations here recorded upon the structure, identified 

 as the thymus-elemeut of the spiracle in Raja, it is seen to agree in 

 its history with an ordinary thymus-element in the following points : 



1) It arises as a placode of the gill-pouch, and with the rupture 

 of this latter it comes to be "epiblastic" in position. 



2) In later phases, corresponding to those in which the like hap- 

 pens to an ordinary thymus-element, its base acquires a covering 

 of ordinary epiblast. 



3) In some cases, if not in all, connective tissue septa grow 

 into it. 



4) Blood-capillaries penetrate it. 



5) Its epithelial cells give origin to leucocytes. 



6) At a later period it becomes more or less constricted oiï from 

 the branchial epithelium, but, apparently, unlike an ordinary 

 thymus - element, not completely. In this latter respect it 

 resembles the thymus-elements of Gasterosteus (Maurer), and 

 of Retromyzon, which, according to the researches of Schaffer 

 (1894) never become separated from the branchial epithelium. 



That is to say, except in the trifling final point, it passes through 

 all the evolutions of a true thymus-element. 



In the final phases of the body here dealt with, it corresponds 

 in structure and position with Van Bemmelen's ventral follicle of the 

 spiracle. From his account already cited it appears clear, that Van 

 Bemmelen's researches upon it begin at about the period where mine 

 cease, or that the latter reach to the point of its highest degree of 

 development. It is strange, though not inexplicable, that he found no 

 leucocytes within it. This failure may be due to their possible ab- 

 sence in it in later phases, or it may have arisen from the poverty 

 of the material of Raja at his disposal. And, as already stated, though 

 he does consider in detail the evidences pointing to its thymus-uature, 

 he finally rejects these in favour of the supposition, that it may re- 

 present the rudiment of a gill-cleft. 



Undoubtedly, the spiracular thymus of Raja is a rudimentary 

 structure, of more morphological than physiological import. 



It is of interest in many ways. Its presence realises the theoretical 

 number of thymus-elements in the existing gill-region of Raja. And 

 at present it places Raja in the position of being the vertebrate 



