The origin and histogenesis of the thymus in Raja batis. 465 



one ought to speak of the young forms as foetuses. The conversion 

 of the epithelial cells into leucocytes is described and figured exactly 

 in Kölliker's sense. The origin of the concentric corpuscles 

 did not fall within the scope of Prenant's researches, but he is in- 

 clined to regard them as derived from the remains of the original 

 epithelium. 



To the same year belongs my own first paper i) upon the thymus, 

 in which for the skate the development and histogenesis are stated 

 to confirm Kölliker's conclusions. 



In my communication among other things the apparent absence 

 of a thymus in the lamprey was commented upon as an inexplicable 

 circumstance. In 1894 Schaffer was able to fill in this gap in our 

 knowledge by the description of an extensive thymus in Ammocoetes. 

 According to his account, which from subsequent observation I can 

 confirm, there are seven thymus-elements on each side in this animal, 

 and in two respects these difier from those of other vertebrates yet 

 described. As in Gasierosteus, according to Maurer, the thymus- 

 elements of the lamprey do not become disconnected from their places 

 of origin, and as in no other known vertebrate they are represented, 

 not at the dorsal aspects of the gill-pouches only, but also at the 

 ventral ones. 



Recently an attempt has been made by Maurer (1899, p. 167) to 

 place a different interpretation upon Schaffer's finds, in that he 

 would homologise the ventral portions of the thymus-elements of the 

 lamprey with the so-called "epithelial bodies", which are found in 

 connection with the thymus from the Anura to the mammalia ^), while 

 the dorsal elements of the lamprey would be the complete homologues 

 of those of higher vertebrates. Regarding the latter point there is 

 no doubt at all, but the former conclusion may be challenged upon 

 the following grounds. Maurer has failed to study these structures 

 in the lamprey for himself, and, therefore, his opinion is a mere hypo- 

 thesis, the ventral thymus-elements of the lamprey are in struc- 



1) In that paper, of which I never saw a proof, on p. 486 the 

 last part of the first paragraph should read: "As the guardian of the 

 respiratory organs the thymus is functionally relieved b}'^ the tonsils 

 (palatine, etc.) in the Sauropsida and Mammalia, the change in the 

 protecting organ following that in the organs of respi- 

 ration." 



2) For an excellent account of our knowledge of these structures 

 see : Alfred Kohn, Die Epithelkörperchen, in : Merkel u. Bonnet, Er- 

 gebn. Anat. Entw., V. 9, 1900, p. 194 — 252. 



Zool. Jahrb. XVII. Abth. f. Morph. gQ 



