xii CONTENTS 



CHAPTER . PAGE 



3. Spermatogenesis 244 



4. Oogenesis 246 



5. Chromosome Cycle 249 



XIX. FERTILIZATION 251 



A. Gametes 251 



B. Union of Gametes 253 



1. Synkaryon 254 



2. Significance of Fertilization ...... 255 



Protozoa 255 



Metazoa 258 



XX. DEVELOPMENT 263 



A. Embryology of the Earthworm 263 



B. Embryology and Metamorphosis of the Frog . . . 267 



C. Embryonic Membranes of the Higher Vertebrates . 271 



D. Problems of Development 274 



XXI. INHERITANCE 281 



A. Heritability of Variations 283 



1. Modifications 285 



2. Recombinations 287 



3. Mutations 287 



B. Mendelian Principles 288 



1. Monohybrids 288 



2. Dihybrids 292 



3. Trihybrids 294 



4. Summary 294 



C. Alterations of Mendelian Ratios 297 



D. Mechanism of Inheritance 301 



1. Sex Determination 304 



2. Linkage 307 



3. Crossing-over 308 



4. Mutations 309 



E. Nature and Nurture 312 



XXII. ORGANIC ADAPTATION 316 



A. Adaptations to the Physical Environment. . . . 316 



1. Adaptations Essentially Functional . . . 317 



Food 317 



Temperature 318 



Pressure 320 



2. Adaptations Essentially Structural .... 320 



Adaptive Radiation of Mammals .... 320 



Animal Coloration 324 



Legs of the Honey Bee 329 



B. Adaptations to the Living Environment .... 335 



1. Communal Associations 336 



2. Symbiosis 337 



3. Parasitism 339 



4. Immunity 342 



C. Individual Adaptability 343 



