EXPLANATION OF PLATES 35, 36. 843 



Fig. 14. Side view of a larva about 15 millims. long, five days after hatching. 

 Fig. 15. Head of a larva 23 millims. in length. 

 Fig. 16. Tail of a larva ir centims. in length. 



Fig. 17. Transverse section through the egg-membranes of a just-laid ovum. 

 We are indebted to Professor W. K. Parker for figs. 12, 14 and 15. 



PLATE 35. 



Figs. 1 8 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 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. 2 1 . 



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. Tranverse 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. 3-2. 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. 



54-2 



