1902.] NATURAL SCIENCES OF PHILADELPHIA. 95 



been pushed iu, at one point, by the lateral invagination (i.v. ). 

 On the medial side of this oval mass of cells, lying parallel to the 

 median lamella (^m.L), is seen a double row of nuclei, the lateral 

 row {n.g.L) apparently having been separated from the medial row 

 (n.f.L). The nuclei in the medial row become the nuclei of short 

 columnar cells (fig. 5f, f.L), while the lateral nuclei remain in 

 the gland cells proper (fig. 5/, g.l). A considerable space is formed 

 between the ciliated cells and the gland cells, which becomes con- 

 nected with the lateral invagination (figs, of-h, i.v. ) by the separa- 

 tion in the centre (at the end of the line n.g.L, fig. be) of the 

 nuclei of the lateral row (n.g.L ), and the joining of these separated 

 ends with the nuclei of the adjacent sides of the invaginated double 

 row of nuclei; the cells above (dorsal to) the line n.g I. (fig. be) 

 joining with those above the invagination (i.v.), and the cells 

 below (ventral to) n.g. I. joining the cells on the lower side of the 

 invagination. In other words, the space formed between the 

 ciliated cells and the gland cells becomes connected with the exte- 

 rior by the breaking through (between the short parallel Imes at 

 the end of the reference line n.g.L, tig. be) of the lateral invagina- 

 tion. The nuclei that are connected by the short black lines, 

 referred to above, will lie hereafter in adjacent gland cells. 



The result of these changes is not only the formation of a dis- 

 tinct layer of columnar ciliated cells (tig. bf,f.L), but also the 

 formation of four groups of gland cells in each half of the thyroid. 

 Of these four groups of cells, two are large and have reached 

 nearly their full differentiation at this stage (figs, be-j, g.r.) ; the 

 other two (figs, be-j, g.r'.) are still very small and undeveloped, 

 though even when fully developed they remain somewhat smaller 

 than the first formed groups. The large groups of gland cells are 

 somewhat fan-shaped, when seen in cross section, and are composeti 

 of greatly elongated conical cells, whose nuclei lie at their bases. 



On account of the great difficulty in obtaining good preparations 

 of these gland cells, as before mentioned, many series of sections 

 had 1o be studied before any conclusion could be reached as to the 

 minute stx'ucture of the cell groups. The pointed ends of the cells 

 are directed toward the opening (o. ) of the cell group, and, in the 

 later stages at least, are usually obscured by the mass of secretion 

 which fills any space there may be in the centre of the cell group. 

 In practically every instance there was a small protuberance at the 



