69. Ovum of the Sea-urchin 281, 338 70. _Daphnella_ 283 71. _Bythotrephes longimanus_ 283 72. _Sida crystallina_, a Daphnid 284 73. Diagrammatic longitudinal section of a hen's egg before incubation 285 74. Diagram of nuclear division 288 75. Process of fertilization in _Ascaris megalocephala_ 296 76. Diagram of the maturation divisions of the ovum 299 77. Diagram of the maturation divisions of the sperm-cell 301 78. Diagram of the maturation of a parthenogenetic ovum 305 79. The two maturation divisions of the 'drone eggs' of the Bee 307, 337 80. Fertilization of the ovum of a Gasteropod 310 81. Formation of polar bodies in a Lichen 313 82. Fertilization in the Lily 314 83. Conjugation of Noctiluca 317 84. Conjugation and polar body formation in the Sun-animalcule 319 85. Diagram of the conjugation of an Infusorian 321 86. Conjugation of an Infusorian 323 87. Diagram to illustrate the operation of amphimixis 348 88. Sperm-mother-cells (spermatocytes) of the Salamander 350 89. Anterior region of the larva of a Midge 364, 393 90. The Common Shore-Crab, seen from below 367 91. Hind leg of a Locustid 371 92. Echinoderm-larvæ 387 93. Development of a limb in the pupa of a Fly 395 94. Diagram to illustrate the phylogenetic shifting back of the origins of the germ-cells in medusoids and hydroids 412 95. Diagram to illustrate the migration of the germ-cells in Hydromedusæ 414 COLOURED PLATES SOME MIMETIC BUTTERFLIES AND THEIR IMMUNE MODELS PLATE I _to face page_ 112 PLATE II " " 114 PLATE III " " 116 LECTURE I INTRODUCTORY