460 JOHN BEARD, 



the like fact is also noted. In an embryo of 22 mm the canal of 

 the tube is hardly any longer to be seen, and the whole organ is 

 surrounded by a connective tissue capsule. In older embryos of 35 mm 

 there is a considerable development of connective tissue around the 

 thymus, and blood-vessels appear. On p. 24 he describes between the 

 sheath [Hülle] and the epithelial thymus aggregations of adenoid cells. 

 "Aber ein anderes Element ist hinzugekommen um die Thymus weiter 

 zu bilden, adenoides Gewebe [? whence] mit Blutgefässen." "Ueber 

 dieses Stadium hinaus", he goes on to say, "habe ich — auffallender 

 Weise — die epithelialen Elemente der embryonalen Thymus mit 

 Sicherheit nicht verfolgen können" [!!!]. Finally, it is added, that in 

 embryos of 50—60 mm the section of the thymus presents a structure 

 similar to that in new-born sheep. 



On p. 24 two questions are asked: "Wo sind die Epithelzellen 

 hingekommen? Woher stammen die kleinzelligen Massen im Innern 

 der Thymus"? To the second question, after rejecting the origin of 

 the small-celled masses from the epithelium , Stieda replies with 

 another query "wie soll das adenoide Gewebe aus der epithelialen An- 

 lage hervorgehen ? Wenn die Zellenmassen der Thymus eines Embryos 

 von 50 mm nicht epithelialen Ursprungs sind, [and the proof of this?] 

 warum sieht man nichts mehr von den früher so überaus scharf und 

 distinct hervorgetretenen Epithelzellen"? Why, indeed! The second 

 question — and it is to be hoped to the satisfaction of the reader 

 — is answered by putting two additional queries. After this the 

 second question is deserted in favour of the consideration of the first. 



In explanation of the disappearance of the epithelial cells the 

 observation, that in embryos of 100 mm "grosse, rundliche, kernhaltige 

 Zellen" are found, is cited, and these are figured in tab. 1, fig. 17. 

 Probably, large cells, markedly different from the ordinary leucocytes 

 of the thymus, have been seen by every investigator of the thymus 

 since 1881, but nobody has as yet observed the transformation of 

 such into concentric corpuscles. The fact, recorded by Stieda, is 

 the occurrence of large cells, described as above, in the thymus of a 

 foetus of 10 cm. Regarding this observation, he writes (p. 25): "Meine 

 sich auf die hier an Schafembryonen mitgetheilten speciellen Unter- 

 suchungen gründende Vermuthung (!) ist nun die, dass jene Zellen 

 modificirte Abkömmlinge der epithelialen Embryonalaulage der Thymus 

 sind. Der Beweis des genetischen Zusammenhangs zwischen den Epithe- 

 lialzellen der embryonalen Thymus (Embryo von 35 mm) und den grossen 

 vereinzelten Zellen eines Embryos von 50 — 60 mm fehlt mir noch." 



