360 



In investigating the early history of the gill-clefts and associated 

 structures, of which study the results are in part described in the 

 account of the development of the thymus, the spiracular cleft itself 

 also came under review. It had long appeared remarkable, that, while 

 its origin and much of its later history were those of a typical gill- 

 cleft, while the equivalent of a thymus-placode was easily made out 

 in embryos of 6 to 10 mm or more, no evidences of a real thymus- 

 element could be encountered. Indeed, it was not until the close of 

 the work on the thymus that such presented themselves. 



The earliest embryos, in which leucocytes were found in the 

 spiracular thymus-placode, were two of about 28 mm (nos. 443 and 

 627). In the latter embryo the spiracular thymus-placode is rather 

 larger in area than that of the first branchial cleft of an embryo of 

 17 mm. It is, however, very simple in character, and consists of but 

 a single regular epithelium of high columnar cells. 



Here and there, especially near the basal side of the epithelium, 

 there is a single leucocyte. In the drawing of one such section of 

 the placode there are in all only three leucocytes. Very similar 

 features are observed in embryo no. 629 (27 mm), but here, while the 

 epithelium is still single, the number of contained leucocytes is rather 

 more, as many as 6 in a section, and there are a few (4 to 6j in 

 each section in the mesoderm to the inner side. 



The result of the examination of many embryos of this and 

 earlier periods is, that while the thymus-placode of the spiracle of 

 Raja arises just like any of the others, from a special portion of the 

 dorsal wall of a gill-pouch, and while its subsequent history after the 

 rupture of the pouch up to embryos of 28 mm or thereabouts bears 

 a general resemblance to that of any of the others, it dilfers from 

 them in two respects. 



In the first place, leucocytes are not formed in the epithelium 

 for a very long period; or, if they arise, they do so in such small 

 numbers as to elude observation. In the second, although the epi- 

 thelium grows in area, it does not increase in complexity for a long 

 time. 



In the later history also variations would appear to obtain in two 

 somewhat important respects. 



In embryos of 42 mm to 45 ram the leucocytes within the epi- 

 thehum are more numerous than in embryos of 27 to 28 mm, but 

 they are still not abundant. The epithelium itself is hardly more 



