559 



pouches at a very early stage, before the pouch is open to the exterior. 

 The thymus of the skate is, therefore, of hypoblastic origin. As is 

 well known, Koelliker maintained that in the rabbit the thymus arose 

 from (the wall of) a modified gill-cleft, but its hypoblastic nature and 

 its origin as a modified portion of the hypoblastic lining of a gill- 

 pouch were not clearly determined. Dohrn's researches in Elasmo- 

 branchs begin at a far later period of the development i), than that 

 in which the above results were obtained. His held that in mammals 

 the thymus was of epiblastic origin — a view which is not supported 

 by the researches of Koelliker, Born, and Schultze, and since re- 

 jected by His himself. 



The early Histogenesis of the Thymus and the Origin 



of Leucocytes. 



This period begins as a rule in embryos of about 17 mm. It 

 must, however, be stated, that in one embryo of 10 mm what appeared 

 to be an undoubted leucocyte was detected in a section of the thymus- 

 placode, but generally the latter remains in the epithelial condition 

 until the embryo is 17 — 18 mm long. In embryos below this size 

 there is as a rule an entire absence of leucocytes anywhere else in the 

 body, and, other than the thymus, no lymphoid structures whatsoever 

 are present. 



The examination of about 20 embryos of 10 to 16 mm failed to 

 reveal any leucocytes whatsoever either in the heart, or blood vessels, 

 or mesoblast, or in the thymus-placodes-). 



In one embryo of 20 mm repeated examination of the thymus- 

 placode with the Vi2 iiic'i oil -immersion failed to reveal any fully 

 formed leucocytes within it, although some of the epithelial cells of 

 the structure were taking on what may be termed leucocytic characters. 

 In this embryo there were few leucocytes in the blood and heart. The 

 explanation of the absence of fully formed leucocytes in the thymus 

 here is that the few leucocytes formed had already wandered out. In 

 another embryo of the same size there appear to be one or two leuco- 

 cytes within the thymus-placode, and there are also one or two out- 

 side of it in the mesoblast, as well as a few in the heart. In an em- 

 bryo of 17 mm a single leucocyte was detected in the thymus-placode 

 of the first branchial cleft. Nowhere else in this embryo were any 



1) With embryos corresponding, as nearly as can be judged, to 

 Raja bat is specimens of 26 — 28 mm. 



2) With the solitary exception mentioned above. 



