462 JOHN BEARD, 



webe umwachsen und verdrängt werde, wobei ihre Reste als con- 

 centrische Körper persistireu. Seitdem Kölliker bei 2wöchentlichen 

 Kaninchenembryonen die Thjmusanlagc als ein in der That epitheliales 

 Hohlgebilde nachgewiesen hat, hat meine Annahme von der Bedeutung 

 der concentrischen Körper, wie mir scheint, sehr an Gewicht gewonnen, 

 auch hat sich, seitdem ich dieselbe im ersten Heft ausgesprochen habe, 

 Stieda dafür erklärt." 



He then compares the later thymus to what the mineralogists 

 term a pseudomorph, that is, that it is a substitution-product of the 

 one first laid down. It is suggested, that the thymus must be, like 

 the epidermis, a product of the epiblast — a view since rejected by 

 His himself. The rest of the description relates to the question as 

 to which gill-pouches in man give rise to thymus-elements, viz., the 

 fourth, third, and, partly, the second. 



The reference to a previous portion of his work is to Heft 1, 

 p. 56, 1880, where in a foot-note we read: "Kölliker's Angaben 

 über eine epitheliale Anlage der Thymus stimmen überein mit einer 



Vermuthung, die ich seit längerer Zeit gehegt habe Ich halte 



für selbstverständlich, dass das adenoide Gewebe nicht aus der Epithel- 

 anlage, sondern aus deren Umgebung entsteht; als Reste von jener 

 sind die concentrischen Körper anzusehen." 



In neither of the forgoing works is the origin of the lymphoid 

 elements, or of the concentric corpuscles, established by observation. 

 Neither Stieda nor His attempts to prove, that the former have arisen 

 from the mesoderm, the latter from the original epithelial cells, which 

 according to Stieda's own account have disappeared 

 long before any Hassall's corpuscles appear on the 

 scene. This observation of Stieda's is, indeed, absolutely correct. 

 In other mammals besides the sheep, thus in the rabbit, the entire 

 thymus is apparently lymphoid, long before any Hassall's bodies are 

 seen in it. In a new-born rabbit, for instance, the thymus is in 

 section very like that of a skate of 7 cm. It is filled with leucocytes, 

 epithelial cells seem to be absent, and there are no concentric cor- 

 puscles. 



Stieda's hypothetical views as to the history of the histogenesis 

 of the thymus have been received by the majority of embryologists 

 as though a basis of fact underlay them. Even to-day in most of 

 the leading text-books these views will be found gravely stated as the 

 results of exact research, and in the latest edition of Gegenbaur's 

 "Vergleichende Anatomie" the "fact", that the original epithelial cells 



