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J. D. F. GILCHRIST. 
und 8; figs. 1, 17, and 20 with a Zeiss F objective; figs. 2-13 with 
a Zeiss A, and the remainder with a Zeiss C. The ectoderm i-j 
represented diagramatically by a light shading, the yolk grannies by 
a stippled shading, where details are unnecessary. The scale of 
magnification is shown by a line representing 50 /*.] - 
PLATE 13. 
Fig. 1. — Section of ovarian egg. N. Nucleus, n. Nucleolus, y.gr 
Yolk granules, y.n. Yolk nucleus. 
Fig. 2. — Fertilised ovum. 
Fig. 3. — Two-celled stage with nearly equal division. 
Fig. 4. — Two-celled stage with unequal division. 
Figs. 5-7. — Four-celled stage showing various methods of division. 
Fig. 8. — Three-celled stage. 
Fig. 9. — Six-celled stage. 
Fig. 10. — Section of egg showing 6 blastomeres and segmentation 
-cavity. 
Fig. 11. — Section o£ blastula showing blastoccele and contents. 
Fig. 12. — Section of blastula showing beginning of internal prolifera- 
tion at posterior end. 
Fig. 13. — Section of blastula showing elongation of cells at anterior 
end of embryo. 
Fig. 14. — Section showing a solid embryo, the blastocoele being filled 
with cells from the posterior proliferation. The external cells assume 
the form of external yolk columns (ex. y. c.). 
Fig. 15. — Longitudinal section of an embryo showing the external 
yolk columns in increased numbers, and an invagination at the 
posterior end. 
Fig. 16. — Section of gastrula-like structure showing vitellophags 
(vit), homogeneous detritus ( d .), yolk lumen (y. L), posterior invagina- 
tion (p. inv.), and traces of external yolk columns (ex. y. c.), now 
disappearing at the anterior end. 
Fig. 17. — Section of part of anterior end of embryo showing the 
formation of the endoderm (end.), and the appearance of a basement 
membrane ( b . m.). 
Fig. 18. — Longitudinal section of an embryo showing the formation 
of inner yolk columns ( i . y. c.), the further development of the endoderm 
(end.), and the early appearance of the ventral thickening (v. th.). 
Fig. 19. — Longitudinal section showing further development of 
endoderm, and formation of posterior body cavities (b. c. 2 and b. c. 3 ), 
