XVI 



CONTENTS 



258. Other Endocrine Glands 634 



259. Endocrine Interrelationships 635 



Chapter 31 

 THE DE\'ELOPMEXT OF MAMMALS 637 



260. Early Stages of Mammalian Development 637 



261. Formation of the Xotochord and Neural Tube . 641 



262. The Digestive Tract and Its Derivatives 642 



263. Differentiation of the Mesoderm 643 



264. Growth of the Embryo 646 



265. Twinning 646 



Part IV. Genetics and Evolution 



Chapter 32 



PRINCIPLES OF HEREDITY 649 



266. History of Genetics 649 



267. Mendel's Discoveries 650 



268. Chromosomal Basis of the Laws of Heredity . . . 652 



269. Allelomorphs 652 



270. A Monohybrid Cross 653 



271. Laws of Probability 655 



272. Test Crosses 655 



273. Incomplete Dominance 656 



274. A Dihybrid Cross 656 



275. Problem Solving 658 



276. The Genetic Determination of Sex 660 



277. Sex-Linked Characteristics 662 



278. Linkage and Crossing Over 663 



279. Chromosome Maps 666 



Chapter 33 



GENETICS 669 



280. The Interactions of Genes 669 



281. Multiple Factors 674 



282. Multiple Alleles 677 



283. Lethal Genes 679 



284. Penetrance and Expressivity of Genes 680 



285. Inbreeding and Outbreeding 680 



286. Population Genetics 681 



287. Biochemical Genetics 683 



