class="sn">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 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 55. The same, modified for a bird 151 56. The same, modified for a mammal 151 57. A series of embryos at three comparable and progressive stages of development, representing each of the classes of vertebrated animals below the Mammalia 152 58. Another series of embryos, also at three comparable and progressive stages of development, representing four different divisions of the class Mammalia 153 59. Diagram of geological succession of the classes of the Animal Kingdom 165 60. Skull of Oreodon Culbertsoni 167 61,62. Horns of Cervus dicrocerus 168 63. Horns of C. matheronis 168 64. Horns of C. pardinensis 168 65. Horns of C. issiodorensis 168 66. Horns of C. Sedgwickii 168 67. Successive stages in the development of an existing Deer’s Antlers 169 68. Homocercal tail 169 69. Heterocercal tail 170 70. Vertebrated but symmetrical fin (diphycercal) 170 71. Tail of Archæopteryx 171 72. Tail of modern Bird 171 73.