The origin and histogenesis of the thymixs in Raja batis. 467 



replacing the original epithelial cells, some of which he supposed 

 furnished the concentric corpuscles. From 1880 to 1891 these views 

 influenced all research upon the thymus. In this sense the researches 

 of Stieda, Dohrn, Maurer, and Gulland, and to some extent those 

 of TouRNEUx and Hermann, were written. But by no single one of 

 these investigators was the immigration of leucocytes into the thymus 

 established. All the researches, written since 1891, have gone to con- 

 firm and extend Kölliker's conclusions of 1879. According to these 

 the original epithelial cells of the thymus became converted into 

 leucocytes. This was independently confirmed by Prenant in the 

 sheep and by the writer in the skate in 1894, The confirmation was 

 extended to include the bat by Oscar Schultze in 1897, Echidna 

 and the lizard by Maurer in 1899, and the trout by Nusbaum & 

 Prymak in 1901, 



The idea, that the leucocytes were immigrants, never had any real 

 basis of observed fact in its favour: Kölliker's view, on the other 

 hand, has been confirmed again and again, up to the hilt, in diverse 

 vertebrate animals. 



In the foregoing account of research into the thymus the reader 

 may have noticed the absence of reference to those researches, which 

 treat of its developmental origin from gill-pouches and from which of 

 these, without entering into the question of its histogenesis. In this 

 way the list of the thymus-literature has been considerably reduced. 

 For this course the following reasons may be given. The comparative 

 embryology of the thymus, that is to say, from which of the pouches 

 it takes its origin in this, that, or the other animal, may be found in 

 certain of the text-books of embryology. And, since in 1886 de Meuron 

 initiated this procedure by his researches, comparative tables after 

 his system have been given by several authors. Thus, recently 

 Maurer (1899, 2, p. 98) has given tables of the conditions in certain 

 of the lower vertebrates and in Echidna^ and in the latest memoir 

 upon the thymus, that of Livini^), the author devotes 2 plates to 

 this matter. 



But there is another consideration, which has led me to refrain 

 from the setting-up of such additional tables. In none of the exist- 

 ing ones is a thymus-element shown in connection with the spiracle. 



1) Ferdinand Livini, Organi dei sistema timo-tiroideo nella Sala- 

 mandrina perspicillata, in: Arch. Ital. Anat. Embriol., V. 1, p. 3 — 96, 

 7 tabb., 1902. Compare tab. G and 7. 



30* 



