-17- 



irx whicli it was at the last stage, but as we pass to seotions 

 farther back we find that the invagination forms a oonsiderable 

 cavity in each lateral half of the gland (Figs. 5 f-h, i.v.), 

 and posterior to the duot the gland, in cross section, is seen 

 to be divided into three distinct parts, a central and tv;o lat- 

 eral divisions (Figs. 5 i-k). In the posterior part of the 

 invagination mesoblast cells are seen often (Fig. 5 i, mes,). 



The most extensive and complicated changes that have 

 taken place are those of the gland cells (g.l.). The first among 

 the changes to be mentioned is the separation of these cells 

 into two groups, the ciliated cells, and gland aells proper. 

 The method of this separation is partly shovm in "ig. 5 e. As 

 seen in this section, the nuclei of the gland cells appear to 

 have arranged themselves near the periphery of a somev/hat oval 

 mass of cells and to have been pushed in, 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.gl.) appar- 

 ently having been separated from the medial row (n.f.l.). The 

 nuclei in the medial row become the nuclei of short columnar cells 

 (Fig. 5 f, f.l.), while the lateral nuclei remain in the gland 

 cells proper (Fig. 5 f,g.l,). A considerable space is formed 

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

 nected with the lateral invagination (Figs. 5 f-h, I.v.) by the 

 separation, in the centre (at the end of the line n.g.l., Fig.Se) 



