CHAPTER XXV. 



DEVELOPMENT OF THE URINARY AND SEXUAL ORGANS. 



Importance of Reproduction. — Growth. — Simplest Forms of Asexual Repro- 

 duction: 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 ; Later Separation of the 

 Sexes (Gonochorism). — Original Development of the Two Kinds of 

 Sexual Cells from the Two Primary Germ-layers. — The Male Exoderm 

 and Female Entoderm. — Development of the Testes and Ovaries. — 

 Passage of the Sexual Cells into the Coelom. — Hermaphrodite Rudiment 

 of the Embryonic Epithelium, or Sexual Plate. — Channels of Exit, or 

 Sexual Ducts. — Egg-duct and Seed-duct. — Development of these from 

 the Primitive Kidney Ducts. — Excretory Organs of Worms. — " Coiled 

 Canals " of Ringed Worms (Annelida). — Side Canals of the Amphioxus. 

 — Primitive Kidneys of the Myxinoides. — Primitive Kidneys of Skulled 

 Animals {Craniota), — Development of the Permanent Secondary 

 Kidneys in Amniota. — Development of the Urinary Bladder from the 

 AUantois. — Differentiation of the Primary and Secondary Primitive 

 Kidney Ducts. — The Miillerian Duct (Egg-duct) and the Wolffian Duct 

 (Seed-duct). — Change of Position of the Germ-glands in Mammals. — 

 Formation of the Egg in Mammals (Graafian Follicle). — Origin of the 

 External Sexual Organs. — Formation of the Cloaca. — Hermaphroditism 

 in Man« 



" The most important truths in Natural Science are discovered, neither 

 by the mere analysis of philosophical ideas, nor by simple experience, but 

 •by T^ctive experience, which distinguishes the essential from the accidental 



