CONTENTS 



INTRODUCTION 1 1 



CHAPTER I. THE GERM CELLS 17 



The Ovum 17 



Ovulation and Menstruation 20 



The Spermatozoon 21 



Mitosis and Amitosis 22 



Maturation 24 



Fertilization 30 



The Chromosomes in Heredity 31 



The Determination of Sex 31 



CHAPTER II. SEGMENTATION AND FORMATION OF THE GERM LAYERS 33 



Segmentation in Amphioxus, Amphibian, Bird, and Reptile 33 



Segmentation of the Rabbit's Ovum 36 



Origin of the Ectoderm and Entoderm 37 



Origin of the Mesoderm, Notochord and Neural Tube 42 



Origin of the Mesoderm in Mammals 45 



The Notochord 46 



CHAPTER III. THE STUDY OF CHICK EMBRYOS 48 



Chick Embryo of Twenty-five Hours 48 



Transverse Sections of Twenty-five-hour Chick Embryo 50 



Origin of the Primitive Heart 52 



Chick Embryo of Thirty-six Hours ( 18 segments) 54 



Central Nervous System 54 



Digestive Tube 55 



Heart and Vessels 56 



Mesodermal Segments 61 



Ccelom 63 



Mesenchyma 63 



Derivatives of the Germ Layers 64 



Chick Embryo of Fifty Hours (27 segments) 65 



Nervous System 66 



Digestive Organs 67 



Blood System 67 



Study of Transverse Sections 69 



Amnion, Chorion 74 



Yolk-sac and Allantois 75 



CHAPTER IV. THE FETAL MEMBRANES AND EARLY HUMAN EMBRYOS 77 



Early Human Embryos 77 



Fetal Membranes of Pig Embryos 77 



Umbilical Cord 79 



Fetal Membranes of Early Human Embryos 80 



Chorion 81 



Amnion 83 



Allantois 84 



Yolk-sac and Stalk 86 



Anatomy of a 4. 2 mm. Human Embryo 88 



Central Nervous System 89 



Digestive Canal 89 



Urogenital and Circulatory Organs 92 



Age of Human Embryos 95 



CHAPTER V. THE STUDY OF PIG EMBRYOS 97 



The Anatomy of a six mm. Pig Embryo 97 



External Form 97 



Internal Anatomy 99 



The Study of Transverse Sections in 



7 



