The origin and histogenesis of the thymus in Raja hatis. 407 



die Bedeutung des Organs ist nichts Sicheres bekannt"^). And in 

 the new edition of Gegenbaur's work 2) the destruction of the 

 epithelial foundation by invading leucocytes is stated as a signi- 

 ficant fact. 



The writer has long suspected the function of the organ to be a 

 very important one, but, in spite of repeated attacks upon the problem, 

 investigation of the development furnished nothing beyond a con- 

 firmation of Kölliker's discovery, until the summer of 1898. Even 

 now, when success has completely crowned the failures of past years, 

 at a time when the developmental history and function of the thymus 

 in Baja hatis can be demonstrated on numerous preparations with 

 the utmost certainty and ease, the problem owes its solution in the 

 first instance, not to direct attack upon the organ itself, but to a 

 flank movement. In the course of a systematic investigation of the 

 development of Scyllium canicula^ in aid of which the authorities of 

 the University of Edinburgh had made a grant from the Earl of 

 Moray Research Fund, and, more particularly, whilst noting for sub- 

 sequent publication the periods of origin and degrees of development 

 of all the systems of organs in an extensive series of embryos, a little 

 but significant fact forced itself into notice. 



It was, that for a relatively long period of the development the 

 blood contained only nucleated coloured corpuscles, xanthocytes, as 

 they have recently aptly been named. The circumstance was recalled, 

 that this had originally been noted by Kölliker^) some years ago, 

 and that it had been commented upon by H. Ernst Ziegler^), who, 

 moreover, had conjectured, that the condition persisted, until some 

 lymphoid organ or other arose. The like fact has also been observed 

 by GuLLAND (1891, p. 161). 



The question was asked : "at what period of the development and 

 from what source or sources do the white corpuscles of the blood or 



1) WiEDERSHEiM, R., Grundriss der vergleichenden Anatomie der 

 Wirbelthiere, 4. Aufl., 1898, p. 291. 



2) Gegenbaur, C., Vergleichende Anatomie der Wirbelthiere, V. 2, 

 1901, p. 249. 



3) KöLLiKER, A., Embryonen von Säugethieren und Vögeln, die 

 zu einer gewissen Zeit nur rothe Blutkörperchen enthalten, in: Z. wiss. 

 Zool., V. 40, p. 191. 



4) Ziegler, H. Ernst, lieber die embryonale Anlage des Blutes 

 bei den Wirbelthieren, in: Verh. Deutsch, zool. Ges., 1892, p. 20. 



