Figs. 18-22. Transverse sections of embryo on the third day after
impregnation.
Fig. 18. Through head, shewing the medullary keel.
Fig. 19. Through anterior part of trunk.
Fig. 20. Through same region as fig. 19, shewing a groove (_x_) in the
epiblast, probably artificially formed in the process of
hardening.
Fig. 21. Through anterior part of tail region, shewing partial fusion of
layers.
Fig. 22. Through posterior part of tail region, shewing more complete
fusion of layers than fig. 21.
Figs. 23-25. Transverse sections of an embryo on the fifth day after
impregnation.
Fig. 23. Through fore-brain and optic vesicles.
Fig. 24. Through hind-brain and auditory pits.
Fig. 25. Through anterior part of trunk.
Figs. 26-27. Transverse sections of the head of an embryo on the sixth
day after impregnation.
Fig. 26. Through fore-brain and optic vesicles.
Fig. 27. Through hind-brain and auditory pits.