361 



complex, but it lias increased in area, a change furnishing a key-note 

 to a point in its later history. 



In an embryo of 54 mm (no. 245) the spiracular thymus is not 

 more complex as a whole than the thymus-placode of the first bran- 

 chial cleft in an embryo of half the size. It has fourfold the thick- 

 ness of the simple epithelial spiracular thymus of a 27 mm embryo, 

 but there are not four definite layers of epithelial cells. Its inner 

 and outer sides are bounded by well-marked epithelial layers, while 

 its interior is filled by epithelial cells, lying in all sorts of directions, 

 and here and there a single leucocyte or a small group of such is 

 seen. In addition, as in the normal thymus- elements of embryos of 

 this size, there are blood-capillaries. 



In a similar embryo (no. 239) the spiracular thymus-placode is 

 much thicker — nearly as thick again — but at the same time more 

 circumscribed. Here the increase in area was evidently lacking. It 

 is bounded everywhere by epithelial cells, but in its interior such 

 epithelial elements are entirely wanting. This is made up of leuco- 

 cytes with here and there connective-tissue trabeculae and blood- 

 capillaries. 



A complete contrast is again presented by an embryo of 71 mm 

 (no. 255), In this embryo on both sides of the body the spiracular 

 thymus is represented by a conical structure, possessing a central 

 lumen and projecting into the mesoderm. 



This body is largely epithelial. No trabeculae are developed 

 within it, but it contains one or two blood-capillaries and here and 

 there a few leucocytes. Ordinary covering epithelium has grown over 

 its root or basis next the gill-cavity, as in the cases of the other 

 thymus-elements, indeed, it was noted that the process began at the 

 like period — a phase younger than this — in the thymus-elements 

 of all the clefts and in that of the spiracle. 



The abrupt apex of the cone is made up of a single layer of 

 much elongated epithelial cells, the broader base exhibits two and 

 then several layers of such. 



From the comparison of several embryos the conclusion is war- 

 ranted, that in the later stages of its history the spiracular thymus 

 may develop after one or other of two modes. In some embryos it 

 forms a compact mass, comparable to any of the ordinary thymus- 

 elements of Raja. In others the original placode grows and spreads 

 out as a rather large flattened plate of a single layer of cells. Later 



