CHAPTER XXV. 



DEVELOPMENT OF THE URINARY AND SEXUAL ORGANS. 



Importance of Eeproduction. — Growth. — Simplest Forms of Asexual Eepro- 

 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 Eudiment 

 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 

 Allantois. — Differentiation of the Primary and Secondary Primitive 

 Kidney Ducts. — The Mullerian 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. 



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

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

 by reflective experience, which distinguishes the essential from the accidental 



