620 W. J. SOLLAS. 



Fig. 8. — A single blastomere of Fig. 7. Zeiss, E Obj., No. 4 oc. 



Fig. 9. — Section showing nine segments. 9«, a single blastomere of Fig. 9. 

 Zeiss, D Obj., 4 oc. 



Fig. 10. — A section sliowing nine segments. 



Fig. 11. — Embryo showing traces of a segmentation cavity. 



Fig. 12. — Embryo more advanced, with traces of a segmentation cavity. 



Fig. 13 and 14. — Embryo with numerous blastomeres, but no segmentation 

 cavity. 



Fig. lia. — A single blastomere. Zeiss, E Obj., No. 4 oc. Diameter of 

 this blastomere 0*02 mm., of the nucleus O'OOS mm., of the nucleolus 

 0-002 mm. 



Fig. 15. — Latest stage of the solid morula; the pear-shaped outline is 

 exceptional. 



Fig. 15^/. — A single blastomere of Fig. 15. Zeiss, E Obj., No. 4 oc. 

 Diameter of blastomere O'Ol mm., of nucleus 0'0025 mm., of nucleolus 

 0-0008 mm. 



Fig. 16. — Embryo in which the blastomeres of the morula are beginning 

 to arrange themselves along definite lines. Zeiss, D Obj., 3 oc. ; reduced one 

 half. 



Fig. 16«. — Several cells of the same embryo. Zeiss, E Obj., No. 4 oc. 

 Diameter of cells 0-006 to O'Ol mm., of nucleus O'OOIS mm., of nucleolus 

 00006 mm. 



Fig. 17. — Embryo a stage further advanced, showing the alignment of the 

 cells to form a definite wall surrounding an irregular internal cavity. Zeiss, 

 E Obj., No. 3 oc. ; reduced one half. 



Fig. 17fi. — Constituent cells of Fig. 17. Dimensions: length, 0*012 mm.; 

 breadth, 0-0045 mm. Zeiss, E Obj., No. 4 oc. 



Fig. 18. — Embryo with the external wall definitely constituted. Eoss 

 1-inch Obj., No. 4 oc. 



Figs. 19 to 39, showing the further progress of the embryo, are drawn 

 with Ross's 1-inch Obj. and Zeiss's No. 4 oc, except Figs. 20 and 20a, 24 

 and 24«, 29, 30, and 31. 



Fig. 19. — Section of gastrula, the most normal form observed. 



Fig. 20. — Constituent cells seen en face. Fig. 20«, seen laterally. Zeiss, 

 H Immersion Obj., No. 3 oc. Dimensions: length, 0-01 mm.; breadth, 

 0-004 mm. ; nucleus, 0-003 mm. in diameter. 



Figs. 21 to 23. — Three sections of one and the same gastrula. 



Fig. 24. — Section showing the folding of the wall of a gastrula ; the outer 

 sinuses of the folds represent commencing incurrent canals ; the inner sinuses, 

 derived from the archenteron, the commencing excurrent system. 



Figs. 25 and 25«. — Constituent cells, seen sideways and en face. Zeiss, E, 

 Obj., No. 4 oc. Dimensions of cells : length, 0-008 mm. ; breadth, 0-003 mm. 

 Fig. 26,— Gastrula still further folded. 



