479 



We Dow return to a consideration of the changes that take place 

 in the epithelium of the thymus from the moment that it is prolife- 

 rated off from the apex of the cleft. According to current statements 

 — especially those of embryological and histological text-books — the 

 cell-elements of the thymus retain their epithelial characters after 

 their proliferation is completed, and sooner or later the whole organ 

 becomes infiltrated with lymphoid-elements derived from some unknown 

 "mesoblastic" source. Thus "der ursprüngliche epitheliale Charakter 

 immer mehr verwischt, und durch Lymph-Follikel ersetzt wird" 1 ). 



This is just one of those points in the history of the thymus which, 

 after teaching it for years, I have found to be quite erroneous. 



From the moment of their formation the majority of the cell- 

 elements of the thymus do not bear the least resemblance to epithe- 

 lial cells. In Raja, which from the great number of its thymus- 

 elements must be regarded as a very typical form, they are never 

 connected together in any regular fashion to form a membrane, tube, 

 acinus or other epithelial structure. Their nuclei stain intensely, as 

 others have observed, and from the very start the cell- body, i. e. the 

 protoplasm, is exceedingly scant. The original cells of the thymus 

 are somewhat rounded bodies possessing more the characters of lymph- 

 or adenoid cells than of epithelial ones. 



It has become almost a tradition to speak of the migration of 

 lymph-cells into the thymus in the course of its development. In 

 Koelliker's "Entwickelungsgeschichte" only is a description to be found 

 which, allowing for differences between the developing thymus of a 

 Mammal and that of a fish, would seem to have given the true facts. 

 On p. 878 one may read "Zwischen dem 20. und 23. Tage (in 

 the rabbit) vollzieht sich nun die Hauptumgestaltung des Organes da- 

 durch, daß die Zellen desselben immer kleiner und unscheinbarer werden, 

 bis sie endlich, nachdem auch ihre Grenzen, die früher schon nie be- 

 sonders deutlich waren, ganz sich verwischt haben, wie Ansammlungen 

 kleiner rundlicher Kerne mit wenig Zwischensubstanz erscheinen und 

 der Bau des Organes seinen epithelialen Charakter verloren und den 

 bekannten der Thymussubstanz angenommen hat. Mit dieser Umge- 

 staltung geht eine andere von fundamentaler Wichtigkeit Hand in 

 Hand, nämlich das Einwachsen von Gefässen und Bindesubstanz in die 

 dicken Wandungen des Organes". In this account there is nothing of 



1) R. Wiedebsheim, Grundriß der vergleichenden Anatomie der 

 Wirbeltiere. 3. Aufl., 1893, p. 404. 



