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. 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 impregnation, 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, 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. PLATE 37. Fig. 39. Section through the olfactory involution and part of fore-brain of a larva on the ninth day after impregnation, shewing olfactory nerve. Fig. 40. Section through the anterior part of the head of the same larva, shewing pituitary involution. Figs. 41-43. Transverse sections through an embryo on the eleventh day after impregnation. Fig. 41. Through fore-part of head, shewing the pituitary body still connected with the oral epithelium. Fig. 42. Slightly further back than fig. 41, shewing the pituitary body constricted off from the oral epithelium. Fig. 43. Slightly posterior to fig. 42, to shew olfactory involution, eye, and hyomandibular cleft. Fig. 44. Longitudinal section of the head of an embryo of 15 millims. in length, a few days after hatching, shewing the structure of the brain. Fig. 45. Longitudinal section of the head of an embryo, about five weeks after hatching, 26 millims. in length, shewing the structure of the brain. In the front part of the brain the section passes slightly to one side of the median line.