CONTEXTS. 



Differentiation of the Primitive Intestinal Tube into a Respiratory 

 and a Digestive Intestine. Gill-intestine and Stomach-intestine of 

 the Amphioxus and Ascidian. Origin and Significance of the Gill- 

 openings. Their Disappearance. The Gill-arches and the Jaw- 

 Skeleton. Formation of the Teeth. Development of the Longs 

 from the Swim-bladder of Fish. Differentiation of the Stomach.- 

 Development of the Liver and Pancreas. Differentiation of the 

 Small and Large Intestines. Formation of the Cloaca ... ...311 



CHAPTER XXIV. 



DEVELOPMENT OF THE VASCULAR SYSTEM. 



Application of the Fundamental Law of Biogeny. The Two Sides. 

 Heredity of Conservative Organs. Adaptation of Progressive 

 Organs. Ontogeny and Comparative Anatomy complementary of 

 each other. New "Theories of Evolution" of His. The "En- 

 velope Theory" and the "Waste-rag Theory." Main Germ and 

 Supplementary Germ. Formative Yelk and Nutritive Yelk. Phy- 

 logenetic Origin of the latter from the Primitive Intestine. Origin 

 of the Vascular System from the Vascular Layer, or Intestinal- 

 fibrous Layer. Phylogenetic Significance of the Ontogenetic Suc- 

 cession of the Organ-systems and Tissues. Deviation from the 

 Original Sequence ; Ontogenetic Heterochronism. Covering Tissue. 

 Connective Tissue. Nerve-muscle Tissue. Vascular Tissue. 

 Relative Age of the Vascular System. First Commencement of 

 the Latter ; Cceloma. Dorsal Vessel and Ventral Vessel of Worms. 

 Simple Heart of Ascidia. Atrophy of the Heart in the Am- 

 phioxus. Two-chambered Heart of the Cyclostoma. Arterial 

 Arches of the Selachii. Double Auricle in Dipneusta and Am- 

 phibia. Double Ventricle in Birds and Mammals. Arterial Arches 

 in Birds and Mammals. Germ-history (Ontogeny) of the Human 

 Heart. Parallelism of the Tribal-history (Phylogeny) ... ...348 



CHAPTER XXV. 



DEVELOPMENT OF THE URINARY AND SEXUAL ORGANS. 



Importance of Reproduction. Growth. Simplest Forms of Asexual 

 Reproduction : Division and the Formation of Buds (Gemmation). 

 Simplest Forms of Sexual Reproduction : Amalgamation of Two 

 Differentiated Cells ; the Male Sperm-cell and the Female Egg-cell. 

 Fertilization. Source of Love. Original Hermaphroditism ; 



