The origin and histogenesis of the thymiis in Raja batis. 463 



are destroyed by an invasion of leucocytes is emphasized ^). Even 

 Maurer's desertion of his former standpoint, and his adhesion to 

 Kölliker's views in 1899, did not suffice to convince Geüenbaur of 

 the correctness of the latter. 



Before leaving Stieda's work, a reference must be made to his 

 review ^) of a lecture of Kaschtschenko's. His criticism is as follows : 

 "Was die Umwandlung des Epithelgewebes der Thymus in das reti- 

 culäre Gerüst des eigentlichen Thymusgewebes betrifft, die von vielen 

 Autoren — auch vom Referenten — mit aller Bestimmtheit geleugnet 

 wird, so ist das keineswegs auf Grund von theoretischen Erwägungen 

 geschehen, wie Herr Kaschtschenko meint, sondern eben auf Grund 

 eingehender Beobachtungen und Untersuchungen." 



In how far this description of his own earlier work and results 

 tallies with the account of them given above may be left to the 

 reader's decision, 



Born's work (1883) hardly interests us here, for it is concerned 

 mainly with the question as to which pouch or pouches in mammals 

 are the seat of formation of thymus- elements, Dohrn (1884) was the 

 first to show, that the thymus of fishes arose from the epithelium of 

 several pouches^ and in Raja from all five branchial pouches. He 

 accepted Stieda's views of the mesoblastic origin of leucocytes without, 

 so far as can be gathered from his description, which is more morpho- 

 logical than histogenetic, contributing any facts on this point. Dohrn's 

 observations, as elsewhere already insisted, reveal nothing concerning 

 the histogenesis of the thymus, for they begin at too late a period, 

 in embryos corresponding to those of R. hatis of about 28 mm. 

 Moreover, as the writer is aware from observations upon those animals, 

 ScylUum and Fristiurus form nothing like so favourable a material 

 as Raja. 



To the years 1885 to 1888 belong Maurer's earlier works upon 

 the development and histogenesis of the thymus of fishes and am- 

 phibians. These appeared to him to confirm Stieda's views. It is 

 not proposed to cite them here, for the author has seen fit to relinquish 

 his earlier standpoint in favour of that adopted by Kölliker. In 

 fact, Maurer's more recent researches upon the thymus of Lacerta 

 and Echidna lead him to conclusions identical with those of the latter. 



1) Gegenbaur, C, Vergleichende Anatomie, V. 2, p. 240. 



2) Stieda, L., Bericht über die anatomische, histologische und 

 embryologische Literatur Russlands (189G — 97), in: Ergebn. Anat. Entw., 

 V. 6, p. 530—693, 1898, 1. c. p. 659. 



