556 



first appearance in the mesoblast; but, as will be apparent in the sequel, 

 the one epoch practically covers the other. Almost as soon as leuco- 

 cytes arise at all, some of them make their way into the blood. 



For a time this seemed to be all that was necessary, and efforts 

 were now directed to a close study of the details of the histogenesis 

 and to the making of careful figures. Of the latter upwards of seventy 

 have been prepared, and if it were needful, ten times that number 

 could be added to them. Later a desire arose to see how far back 

 to earlier stages the origin of the thymus-epithelium could be carried. 

 In working this out it seemed desirable, in order to save time, to con- 

 fine one's attention to the thymus -element of some particular cleft. 

 As, owing to its proximity to the auditory organ, the first branchial 

 cleft is always more quickly found, than any of the other four, it was 

 chosen as the cleft, whose early history should be examined. Another 

 reason weighed in the sale, it was that the first thymus-element, i. e. 

 that of the first branchial cleft, is, for some little time at any rate, 

 rather larger than any of the others. At a later period the second 

 exceeds the first in size. 



Starting with embryos of 17 — 18 mm, in which the first leuco- 

 cytes were formed and forming in the thymus-epithelium, it was found 

 that the latter could be identified in embryos of all stages from 17 mm 

 to one of 10 mm (No, 143), in which the first branchial cleft had re- 

 cently opened. But even this did not completely satisfy curiosity, 

 and careful search in still earlier embryos revealed, that the modified 

 piece of epithelium or placode ^) whose destiny it was to give rise to 

 one element of the thymus, could be clearly made out in an embryo 

 of 6 mm in length. This embryo (No. 135) is the earliest, to which 

 I have yet followed the thymus, and, reversing the order of the re- 

 search, it will be made the starting point for the following description. 



A word or two may be added as to the material, and the way 

 it was treated and examined. The extensive material employed has 

 been preserved in various ways. The best embryos are undoubtedly 

 those treated with Rabl's picro-platino-chloride fluid, or with corrosive 

 sublimate. Osmic acid, in whatever form employed, does not in my 

 experience yield preparations of value in researches on thymus-devel- 

 opment or on leucocytes. The best stain has been found to be picro- 

 carmine. Some of the better preparations are haematoxylin ones, but 



1) To use VON Kri'FFER's convenient term for a small piece of modi- 

 fied epithelium. (Vide Studien zur Entwicklungsgeschichte des Kopfes 

 der Kranioten, Heft 2, p. 64—65.) 



