PLATE II. 



All figures magnified 400 diameters, ectoderm uppermost. 



Figure 7. Transverse section of a very young germinal disk, showing one cell 

 at the centre projecting above the general level of the ectoderm, 

 but clearly an ectodermal cell. 



" 8. Section immediately succeeding that of Fig. 7, and showing essen- 

 tially the same phenomena. 



" 9. More anterior section of the same disk showing the even surface of 

 the ectoderm and the general arrangement of the cells. No " Deck- 

 schicht " cells. 



" 10. Longitudinal section, nearly median, through the germinal disk of 

 Fig. 1. At the left hand or posterior end is seen an early stage in 

 the development of the bridge, which here consists of two layers of 

 cells, the outermost resembling the extra-germinal ectoderm, the 

 inner the germinal ectoderm. Note the dividing nucleus at the 

 base of the bridge. 



" 11. A nearly median longitudinal section of another germinal disk in a 

 little later stage of development. 



" 12. More lateral section through the same embryo as Fig. 11, showing 

 lateral continuity of the bridge and the cavity beneath it. 



" 13. A still more lateral section of the same disk as the two preceding, in 

 which the cavity has disappeared, but the bridge cells are readily 

 distinguished from the true ectodermal cells of the disk. 



