en 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 gastraea-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

51. Diagram of heart and gill-arches of a fish 150

52. One gill-arch, with branchial fringe attached 150

53. Diagram of heart and gill-arches in a lizard 150

54. Ideal diagram of primitive gill-or aortic-arches 151