344 A - RICHARDS AND JAMES T. THOMPSON. 



PLATE II. 



FIG. 8. A sex-cell from the so-hour embryo (B' 23). Showing in detail 

 the region at the edge of the embryo, where the germ layers meet the peri- 

 blast. The sex-cell is in the edge of the gut endoderm, entirely removed 

 from any connection with the lateral mesoderm, and is partially imbedded in 

 the periblast. Several very early cells have been found in this relation to the 

 periblast, but so far it has been impossible to establish any significance to this 

 fact. Due to exertion of unequal tension upon the cell membrane, the cell 

 outline appears slightly irregular. X 1400. 



FIG. 9. Semi-diagrammatic transection of the 46-hour embryo B' 21 X 

 taken at the position indicated by the line " C " in Fig. 20. At this early 

 stage the lumen of the gut is not formed completely, even in this most ante- 

 rior region. The sex-cells lie between the periblast and the lateral mesoderm, 

 at the side of the developing hind gut. X 300. (The positions of the sex- 

 cells in the germinal path correspond to certain stages in development of 

 the gut.) 



FIG. 10. Transection of embryo B' 21 X taken at the position indicated 

 by the line B in Fig. 20. Here the gut endoderm and the lateral mesoderm 

 are differentiated to a certain extent, although the splitting of the endodermal 

 cell mass has not yet occurred. The sex-cell lies above the periblast in the 

 edge of the cell mass. X 300. 



FIG. ii. Transection of embryo B' 21 X taken at the line "A " in Fig. 20. 

 The sex-cell illustrated is in the extra-embryonic region, associated closely 

 with the peripheral endoderm. X 300. 



FIG. 12. A longitudinal section through the elongating tail of the 46-hour 

 embryo B' 21 L. The 4 sex-cells illustrated are in the peripheral endoderm 

 at the extreme posterior of the embryonic area. X 300. 



