-18- 



of the nuclei of the lateral row (n.gl.) and the joining of 

 these separated ends with the nuclei of the adjacent sides of 

 the Invaglnated, double row of nuclei; the cells above (dorsal 

 to) the line n.g.l. (Fig. 5 e) joining with those above the 

 invagination (l.v.), and the cells below (ventral to) n.g 1. 

 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 exterior by the break- 

 ing through (betv/een the short parallel lines at the end of the 

 reference line n.gl., ^ig. 5 e) of tr.e lateral invagination. 

 The nuclei that are connected by the short black lines, referred 

 to above, will lie hereafter in adjacent gland cells. 



The resialt of these changes is not only the formation 

 of a distinct layer of columnar ciliated cells (Fig. 5 f, f.l.), 

 but also the formation of ^'our 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, 5 e-j, g.r.); the other two (Figs. 5 e-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, 

 v/hen seen in cress section, and are composed of greatly elon- 

 gated conical cells, whose nuclei lie at their bases. 



On account of the great difficulty In obtaining good 



preparations of these gland cells, as^f-ore mentioned, many series 



\ 



