476 



das Septum durchaus nicht immer zu einer Platte verwächst, sondern 

 daß auch bei ihnen wie beim Menschen ein Spatium septi pellucidi 

 bestehen kann. 



Eine ausführliche Arbeit, welche die hier berührten Punkte be- 

 handelt, ist abgeschlossen und wird in nicht zu langer Zeit erscheinen. 

 Zürich, 27. April 1894. 



Nachdruck verboten. 



The Development and probable Function of the Thymus. 



By J. Beard, 

 Lecturer on Comparative Embryology in the University of Edinburgh. 



There is perhaps no organ in the Vertebrate body which is en- 

 shrouded in such morphological darkness as the thymus. And this 

 notwithstanding the numerous attacks investigators have made upon 

 it. Every lecturer on the Vertebrata has had to admit to his hearers 

 that nothing whatever as to its nature is known. But it is an organ 

 which the investigator of piscine development is bound to notice, if 

 only on account of its peculiar reaction to staining reagents. 



To myself it has always been the most puzzling organ in the 

 Elasmobranch embryo; for, while explanations have indeed been forth- 

 coming, their nature was such that acceptance, even as possibilities, 

 seemed out of question. Its relations to the gill-clefts, suspected by 

 Remak, were first proved by A. von Koelliker, and at a later period 

 Dohrn was able to demonstrate its development in Elasmobranchs 

 from the epithelium of all the functional gill-clefts. Still more re- 

 cently the important memoirs of de Meuron and Maurer appeared. 



The thymus is an organ whose morphology I have more than once 

 attacked, only to have to admit myself defeated. Its relations to the 

 gill-clefts were ever held prominently in view as factors of probably 

 great morphological importance, and I have repeatedly told my students 

 in past years that in all probability the thymus-problem would be 

 solved with the solution of the ancestral history of the gill-clefts 

 themselves. 



Renewed observations have been made during the past year, and 

 with the great range of stages at my disposal, from an extensive series 



