20. Human ear 86 21. Foetus of an Orang 87 22. Vestigial characters of human ears 88 23. Hair-tracts on the arms and hands of Man, as compared with those on the arms and hands of Chimpanzee 90 24. Molar teeth of lower jaw in Gorilla, Orang, and Man 93 25. Perforation of the humerus (supra-condyloid foramen) in three species of Quadrumana where it normally occurs, and in Man, where it does not normally occur 95 26. Antlers of stag, showing successive addition of branches in successive years 100 27. Fission of a Protozoön 107 28. _Hydra viridis_, partly in section 111 29. Successive stages in the division of the ovum, or egg-cell, of a worm 113 30. Ovarian ovum of a Mammal 121 31. Amoeboid movements of young egg-cells 122 32. Human ovum, mature and greatly magnified 123 33. Stages in the formation of the polar bodies in the ovum of a star-fish 125 34. Fertilization of the ovum of an echinoderm 126 35. Fertilization of the ovum of a star-fish 127 36. Karyokinesis of a typical tissue-cell (epithelium of Salamander) 129 37. Study of successive changes taking place in the nucleus of an epithelium-cell, preparatory to division of the cell 131 38. Formation and conjugation of the pronuclei in _Ascaris megalocephala_ 132, 133 39. Segmentation of ovum 135 40. The contents of an ovum in an advanced stage of segmentation, drawn in perspective 135 41. Formation of the gastrula of _Amphioxus_ 137 42. Gastrulation 138 43. Gastrula of a Chalk Sponge 139 44. _Prophysema primordiale_, an extant gastræa-form 140 45. Ideal primitive vertebrate, seen from the left side 143 46. The same in transverse section through the ovaries 144 47. _Amphioxus lanceolatus_ 145 48. _Balanoglossus_ 148 49. A large Sea-lamprey (_Petromyzon marinus_) 148 50. Adult Shark (_Carcharias melanopterus_) 149