EXPLANATION OF PLATES. 



MAGNIFICATION. 



Fig. 1, Plate V, X 38; all other surface views X 73. 

 All sections X 114. 



REFERENCE LETTERS USED IN THE FIGUEES. 



a-b, c-d. Planes in which were cut sections shown in Plates VII and VIII. 

 b. Bud. 



c. p. Central periblast. 



d. h. ^^ Dis'que hulleu.r." 



0. In fig. 20, furrows not visible in fig. 21. 

 0. p. Outer periblast. 

 ]). b. Protoplasmic bridge. 

 s. c. Segmentation cavity, 

 s. g. c. Sub-germinal cavity. 

 V. Vacuole. 

 ves. Vesicle. 

 X. Lower limit of cells in figs. 84-88. 



Plate V. 



Fig. 1. Egg in shell, blastodisc resting on ''dlsque huileux" which covers the upper 

 third of the yolk. 



2. Two-celled stage, blastomeres equal. 



3. Two-celled stage, blastomeres unequal, vacuole in plane of division. 



4. Four-celled stage, regular, segmentation cavity present. 



5. Irregular 4-blastomere stage. 



6. Four-blastomere stage, reniform, segmentation cavity absent. 



7. Normal 8-celled blastoderm. 



8. Eight-blastomere stage, slightly irregular. 



9. Irregular 8-celled blastoderm. 



10. Eight-celled blastoderm formed by equatorial furrow. Cells 4—4. 



11. Irregular 8-celled blastoderm with equatorial furrow. Cells 2-6. 



12. As above. Two upper cells smaller and shifted to one end. 



Plate VI. 



13. Irregular 8-celled blastoderm, with one cell in center. 



14. Seven-celled blastoderm, equatorial furrow cutting off 3 upper cells. 



15. View of same from below, vertical furrows only visible. 



16. Normal 16-celled stage, cells on one level. 



17. Normal 16-celled stage, central cells slightly raised. 



18. Irregular 16-celled stage. Cells in two layers, blastoderm thicker at loyver 



edge. 



19. Irregular 16-celled blastoderm. Cells in two layers, blastoderm highest in 



center. 



20. Irregular 12-celled stage, derived from a form like fig. 14. 



21. View of same blastoderm from below, showing small segmentation cavity. 



497 



