481 



comes apparent that a different significance is attached to the terra. 

 The expression "lymphoid aussehende Zellen der epithelialen Anlage" 

 would seem to indicate that Maurer does not regard these cells as 

 true lymph-cells. They would have the appearance of lymph-cells, 

 and possibly function as such for a time, only to be replaced, when 

 their powers of proliferation were exhausted, by true lymphoid cells. 

 According to him these latter burst into the thymus and set up lymph- 

 follicles within it. 



Dohrn describes a corresponding wandering in of lymph-cells at 

 the very outset of the development l ). 



I must express dissent from the statements of both observers. 

 Indeed, all I have seen leads me to deny that any such migration 

 takes place at any period. In the course of the work the thymus 

 has been followed in its development from the very start to very ad- 

 vanced stages, in many of which it bears the closest resemblance in 

 structure to a lymph gland. 



Since in Raja bat is one can follow the original epithelial cells 

 from the moment they lose their epithelial characters on proliferation 

 from the dorsal wall of the cleft, until and after the structure of the 

 mature thymus is acquired 2 ), the conclusion to be drawn is naturally 

 that the lymph elements of the thymus are the direct 

 offspring of the epithelial cells of a gill-cleft. And this, 

 as I understand him, is the view Professor von Koelliker took 3 ). 



In other words we have here the origin of a leucocyte-forming 

 structure from the hypoblastic epithelium of gill-clefts. 



1) Gtjlland (Rep. Roy. College of Physicians. Edinb., Vol. Ill, 1891) 

 also believes that the original epithelial foundation becomes substituted 

 or replaced by adenoid tissues. 



2) Except that no concentric corpuscles are yet present. — The nature 

 of these will be discussed subsequently. 



3) Several weeks after the completion of my manuscript I found the 

 recent instructive memoir of A. Pbenant in the current number of "La 

 Cellule" (Contribution ä l'etude du developpement organique et histo- 

 logique du thymus etc., T. 10, Fasc. 1, 1894). The paper treats of the 

 development of the thymus of Mammals, i. e. the sheep, and the author, 

 like myself, is led by his studies to confirm Koellikeb's statements as to 

 the transformation of the original epithelial elements of the thymus into 

 lymphoid cells (see p. 143). Prenant's work affords a valuable and wel- 

 come complement to my own, for whereas he has concerned himself largely 

 with a detailed account of the histological development, the morphological 

 aspect of the thymus problem has been to myself of paramount interest. 

 For further information reference must be made to the original. 



