The origin aud histogenesis of the thymus in Raja batis. 437 



Since Maueer (1899, 1, p. 156) deserted the standpoint maintained 

 by him for so many years, and adopted that originally due to Köl- 

 LiKER, one must go back nearly 11 years in the history of research 

 upon the thymus to find an investigation, in which the ideas of Stieda 

 (1881) and His (1885) are advocated. All recent observers agree in em- 

 phasizing the correctness of Kölliker's original conclusion, and, as 

 ScHULTZE (1897, p. 381) earnestly urges: "Es ist, nachdem Kölliker 

 schon vor langer Zeit die Art der Umwandlung des epithelialen Or- 

 gans in ein bindegewebiges so zu sagen klar ausgesprochen hat, nun- 

 mehr Zeit, dass man sich, frei von Vorurtheilen und Schematismus 

 bezüglich der Leistungsfähigkeit und Bedeutung der Keimblätter, nicht 

 verhehlt, dass hier epitheliale Zellen von noch hochgradiger Differen- 

 zirungsfähigkeit die von uns als Bindegewebszellen betrachteten Ele- 

 mente bilden, und dass epitheliale Drüsenschläuche direct zu 'Lymph- 

 follikeln' werden." 



The writer, indeed, goes further than Schultze and any other 

 observer in stating, not only that the production of leucocytes is a 

 function of the thymus, but also that it must be regarded as the sole 

 original source of all the leucocytes of the body. This latter con- 

 clusion he believes to be a legitimate inference from observations upon 

 a peculiarly favourable material and from certain other considerations, 

 already advanced elsewhere, and to be once more mentioned in the 

 following pages. 



Undoubtedly, the thymus of Baja is the first source of its leuco- 

 cytes, and from the nature of the case a proof of the existence of one 

 or more others is impossible. 



IX. The History of the spiracular Thymus-Placode. 



A new feature of interest to the embryologist, revealed by the 

 present research, is the existence in Raja of a spiracular thymus. 

 Such a structure has been searched for before, not only by the writer, 

 but also by others. One observer. Van Bemmelen, has actually 

 described it, but its rudimentary and somewhat variable characters 

 were probably the things, which led him to reject its thymus-nature. 

 This will not appear at all astonishing; for, as the following description 

 of its history will reveal, it is only by a complete knowledge of its 

 development that its true nature can be elucidated. 



To the structure, identified by me as the spiracular thymus. Van 

 Bemmelen (1885, p. 174 etc.) gave the name of "the ventral follicle of 

 the spiracle". It will be so described here, although it does not lie 



