555 



Nearly all the more important literature is contained in the appen- 

 dices of the most recent German and English editions of Wieders- 

 heim's Comparative Anatomy. The recent researches of 0. Schultze 

 will be found in his "Grundriß der Entwicklungsgeschichte", 1897, and 

 A. Prenant's memoir was published in "La Cellule", Vol. X. This 

 latter contains an extensive but not quite complete bibliography. 



As published now my results concern only one vertebrate animal, 

 R. batis. None the less, what seem to be convincing reasons can be 

 given for extending these to all true Vertebrata and to man himself. 



It may seem rash to draw this conclusion, but it is certain, that 

 the statements regarding the histogenesis of the thymus and the origin 

 of leucocytes, current in most of the leading text-books, have never 

 as yet been established in fact for any single animal. 



It is, surely, hardly necessary to consider the possibility of fun- 

 damental differences in function in the thymus of fishes and of mammals. 



Plan of the Ptesearch. 



For purposes of description there would be no convenience in 

 giving an account of the observations in the order in which they were 

 originally made. After the period, at which leucocytes first appeared 

 in the embryonic blood, had been determined, later stages of the thymus 

 were studied. It seemed preferable to work backwards from embryos, 

 in which the thymus had obtained some considerable degree of histo- 

 logical differentiation, to earlier ones, in which it was a mere piece 

 of epithelium above each of the true branchial clefts. 



It was soon seen, that the migration of leucocytes from the various 

 thymus-elements ^) of the skate into the surrounding mesoblast was a 

 very conspicuous phenomenon. This was found to be especially true 

 of all embryos, whose sizes lay between 25 and 35 mm (greatest 

 length). 



Working backwards again from such stages one could carry the 

 origin of leucocytes down to embryos of 15—17 mm, whilst the thymus- 

 epithelium itself could be traced to still earlier stages. 



The two points, that at which leucocytes first appeared in the 

 mesoblast and in the blood on the one hand, and that at which they 

 arose within the epithelium of the thymus on the other, were thus 

 brought into connection. 



At first no particular attention was paid to the question of their 



1 ! There are five of these on each side in the skate, and of these 

 the fifth is always very small. These facts were first noted by Dohrn. 



