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Nachdruck verboten. 



A Thymus-Elemeiit of the Spiracle in Raja. 



By J. Beard, D. Sc, 

 University Lecturer in Comparative Embryology, Edinburgh. 



The morphological nature of the spiracle as a reduced gill-cleft 

 is probably accepted by all embryologists. But, although its anterior 

 wall often bears a metamorphosed gill, the pseudobranch; although in 

 early life external gills may project from this cleft (the Scylliidae) as 

 from the other true gill-clefts; and, lastly, although the developmental 

 origin and early history of the spiracular cleft and hyoid arch are 

 those of a true branchial cleft and arch, no thymus-element has ever 

 been traced to this region. 



Such a structure has been searched for by two observers at least, 

 by J. F. Van BemxMelen in 1884 and by myself some ten years later. 

 Van Bemmelen, indeed, was within an ace of its discovery. He in- 

 vestigated and described with care the actual structure, only to reject 

 its thymus-character. The negative result is probably to be ascribed 

 to the unfavourable nature of the material and to the rudimentary 

 character of the element itself. The latter reason also accounts for 

 my own previous failures to obtain positive results. 



It will be more convenient to refer to Van Bemmelen's record 

 of his finds after the description of my own recent investigations, for 

 these latter lead to the identification of the undoubted but rudimentary 

 spiracular thymus of Raja with a structure, described by him under 

 the name of the ventral spiracular follicle. 



Although in my previous paper the actual existence of a spiracular 

 thymus could not be established, it was recognised that the ventral 

 follicle of the spiracle in all probability represented such a structure. 

 After a short account of Van Bemmelen's researches and conclusions 

 the following passage occurs: "I incline certainly to the view, which 

 would consider both the vesicular follicle of the spiracle and that of 

 the angle of the mouth as possibly the rudimentary equivalents of 

 thymus-elements of these parts. At any rate, the whole history of 

 these structures harmonises with this supposition" i). 



1) J. Beard, The Development and probable Function of the Thymus. 

 Anat. Anz., Bd. 9, p. 478: 



