KEY TO PLATE. 



Fig. 1. — Mature oocyte lying behind gastral epithelium ; nucleus at beginning 

 of first maturation division. 



Fig. 2. — 0^•um after extrusion of first polar body showing pale nucleus and 

 deeply stained nucleolus; attaching pseudopedium present. 



Fig. 3. — Ovum with nucleus at beginning of second maturation division. 



Fig. 4. — Ovum showing first polar body formed and second one cut oS from 

 nucleus. 



Fig. 5. — Ovum after extrusion of second polar body. Two nucleoli are 

 present as is not infrequently the case in ovum of G. comjjvessa. 



FIG. 6. — Embryo at beginning of segmentation; the two-cell stage. 



Fig. 7. — Section of blastula. 



Fig. 8. — Longitudinal section of embryo after differentiation into columnar 

 cells and smaller number of granular cells at posterior pole. 



FIG. 9. — Section of slightly older embryo ; granular cells beginning to be 

 invaginated into segmentation cavity. 



Fig. 10. — The pseudogastrula in longitudinal section; granular cells increas- 

 ing in number. 



Fig. 11. — Section of older embryo just before being set free; granular cells 

 evaginated and difi'erentiated into single outer layer and inner mass of 

 nutritive cells. 



Fig. 12. — Portion of wall of flagellate chamber showing choanocytes, two of 

 which have become rounded off as future germ cells ; remains of collar 

 still present. 



Fig. 13. — Section of embryo with remains of polar bodies (doubtful) at 

 anterior pole. 



Fig. 11. — Part of flagellate chamber and iuhalent canal showing position of 

 germ cells slightly older than those in Fig. 12 ; larger one with charac- 

 teristic nucleus and nucleolus. 



