STRUCTURE AND DEVELOPMENT OF LEPIDOSTEUS. 
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PLATE 22. 
Figs. 18-22. Transverse sections of embryo on the third day after impregnation. 
Fig. 18. Through head, showing the medullary keel. 
Fig. 19. Through anterior part of trunk. 
Fig. 20. Through same region as fig. 19, showing a groove (x.) in the epi- 
blast, probably artificially formed in the process of hardening. 
Fig. 21. Through anterior part of tail region, showing partial fusion of 
layers. 
Fig. 22. Through posterior part of tail region, showing more complete 
fusion of layers than fig. 2 f. 
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. 
PLATE 23. 
Figs. 28-29. Transverse sections of the trunk of an embryo on the sixth day after 
impregnation. 
Fig. 28. Through anterior part of trunk (from a slightly older embryo 
than the other sections of this stage). 
Fig. 29. Slightly posterior to fig. 28, showing formation of segmental 
duct as a fold of the somatic mesoblast. 
tig. 30. Longitudinal horizontal section of embryo on the sixth day after impregna¬ 
tion, passing through the mesoblastic somites, notochord, and medullary 
canal. 
Figs. 31-34. Transverse sections through an embryo on the seventh day after 
impregnation. 
Fig. 31. Through anterior part of trunk. 
Fig. 32. Through the trunk somewhat behind fig. 31. 
Fig. 33. Through tail region. 
Fig. 34. Further back than fig. 33, showing constriction of tail from 
the yolk. 
