The origin and histogenesis of the thymus in Uaja batis. 469 



notwithstanding the correct researches of Kölliker, Prenant, 

 ScHULTZE, and others, is possibly due to the fact, that most of the 

 work done upon it has been carried out in the highest class of animals, 

 the Mammalia. Here at certain, but not early, periods Hassall's con- 

 centric corpuscles are prominent features, and their presence led ob- 

 servers astray. Had the early researches dealt with fishes, such as 

 the skate, in which concentric corpuscles are conspicuous by absence, 

 the history of our knowledge of the thymus would have been a 

 different story. 



Here it is as clear as the light of day, that the original epithelial 

 cells become converted into leucocytes. From the researches of Köl- 

 liker, Prenant, 0. Schultze, and Maurer the like would appear to 

 hold good for mammals also; and, therefore, no valid excuse can be 

 offered for the neglect to recognise the plain and palpable facts. The 

 supposed fact of the immigration of leucocytes from outside does not 

 help the cause of those, who assumed its existence, for nobody has 

 yet observed this process. 



Moreover, how can leucocytes wander into the thymus from the 

 mesoblast, or for any other part of the embryonic body, at a period, 

 when there are none in existence elsewhere? How can this happen, 

 when the very first source of leucocytes in the body is the epithelium 

 of the thymus itself? 



The formation of leucocytes is, therefore, as Kölliker first showed, 

 and as since confirmed by Prenant, Oscar Schultze, Maurer, the 

 writer, and others, a function of the thymus. But, as my researches 

 now prove, one may go further, and state with confidence, that it is 

 the function of the thymus, not merely to form leucocytes, but to be 

 the parent- source of all the leucocytes of the body. 



It is Kölliker's great service to have shown, that leucocytes 

 arise in the thymus from its original epithelial cells ; to Gulland's 

 researches we owe the result, that the first leucocytes are found in 

 the mesoblast in the immediate neighbourhood of the thymus; and, 

 finally, it has fallen to the writer to demonstrate, that the first leuco- 

 cytes form in the thymus from its epithelial cells; and that thus, as 

 may presently be made evident, it must be regarded as the parent- 

 source of all the leucocytes of the body. 



The last link of the chain, binding the whole together, has at 

 length been forged, and that this is so may be proved in another way. 



Is there any other probable source of leucocytes in the vertebrate 

 body? The answer to this query must now be a decided negative, 

 and for the following reasons. 



