EXPLANATION OF PLATES 35, 36. xlvii 



PLATE 35. 



Figs. 18 22. Transverse sections of embryo on the third day after impregnation. 



Fig. 1 8. 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 harden- 

 ing. 



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 impregna- 

 tion. 



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. Tran verse 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 36. 



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, shewing formation of segmental duct 



as a fold of the somatic mesoblast. 



Fig. 30. Longitudinal horizontal section of embryo on the sixth day after impreg- 

 nation, 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- 3^ Through the trunk somewhat behind fig. 31. 

 Fig- 33- Through tail region. 



Fig- 34- Further back than fig. 33, shewing constriction of tail from the 

 yolk. 



Figs. 35 37. Transverse sections through an embryo on the eighth day after 

 impregnation. 



Fig- 35- Through fore-brain and optic vesicles. 



Fig. 36. Through hind-brain, shewing closed auditory pits, &c. 



Fig- 37- Through anterior part of trunk. 



Fig. 38. Section through tail of an embryo on the ninth day after impregnation. 



e 2 



