The origin and histogenesis of the thymus ia Eaja batis. 441 



the body the spiracular thymus is represented by a conical structure, 

 possessing a central lumen, as stated by Van Bemmelen, and pro- 

 jecting into the mesoderm. As in the follicles examined by this author, 

 the structure is largely epithelial. No trabeculae are developed within 

 it — in marked contrast to the follicle in embryo No. 239 — but it 

 contains one or two blood-capillaries, and here and there a few leuco- 

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

 much elongated epithelial cells, among which leucocytes are found; 

 the broader base exhibits two and then several layers of such epi- 

 thelial cells. The lumen has become broken up into two portions by 

 the proliferation of epithelial cells into the cavity. Ordinary covering 

 epithelium has grown over the base of the structure, as ia. 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 also in that of the spiracle. 



If the nature of the spiracular follicle fell to be determined by 

 the study of but such cases as the present one, its thymus-character 

 might well be open to grave doubt. The pictures before Van Bem- 

 melen probably resembled that given in Fig. 64, and he was not so 

 fortunate as to obtain one like Fig. 66. The latter, along with a 

 number of others not drawn, is, however, decisive. 



From the comparison of several embryos, including those figured, 

 the conclusion is warranted, 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 

 other 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 on in the latter this plate becomes invaginated, forming 

 a conical structure with a central lumen, as in embryo No. 255. In 

 the former cases in the later period of the development there is no 

 cone, because the invagination is lacking, but instead thereof there 

 arises a thick knob-like structure, resembling an ordinary thymus- 

 element of Raja. As elsewhere pointed out at length (Beard, 1900, I, 

 p. 362), this difference is of interest, because it corresponds to a like 

 divergence, witnessed in the development of the ordinary thymus- 

 elements of Raja and of ScylUum. The details of the comparison will 



had the form of an oval elongated vesicle, attached by a delicate stalk, 

 and composed of a single layer of epithelial cells. In one or two cases 

 there were a few leucocytes in the epithelium. 



