Elementary text-book of zoology (1884) Elementary text-book of zoology elementarytextbo0201clau Year: 1884 ; <)|{<; \\s. 127 —usually unite to form an unpaired terminal section—the urethra, which, in Teleusteans only, opons behind the anus; very often it opens into the cloaca, and iii Mammals almost always unites with the terminal parts of the genital ducts to form a common urogenital canal. A vesicular reservoir—the urinary Madder—is often inserted into the course of the efferent ducts. In fishes only does the bladder lie behind the intestine. Reproduction is always sexual,
Elementary text-book of zoology (1884) Elementary text-book of zoology elementarytextbo0201clau Year: 1884 ; <)|{<; \\s. 127 —usually unite to form an unpaired terminal section—the urethra, which, in Teleusteans only, opons behind the anus; very often it opens into the cloaca, and iii Mammals almost always unites with the terminal parts of the genital ducts to form a common urogenital canal. A vesicular reservoir—the urinary Madder—is often inserted into the course of the efferent ducts. In fishes only does the bladder lie behind the intestine. Reproduction is always sexual, and separate sexes are the rule. A few fishes only (species of Serranus) are hermaphrodite. In male Amphibians how- ever traces of ovaries are found. Both kinds of sexual glands lie as paired organs in the body cavity, and send off paired ducts which in the lower Vertebrates open into the cloaca and often join to form an unpaired canal. Sometimes indeed the ducts are absent and the genital products fall into the body cavity and pass out thence to the exterior by a genital pore. The division of the genera- tive ducts into dif- ferent regions, and FIG. 581.—Transverse section through a young embryo of Triton feeniatim (after O. Hertwig). a, First appearance of the medullary folds and formation of the uotochord. b, Closing of the medullary groove. The notochord is completely separated off from the entoderni. The constric- tion of the mesoderm into the protovertebrse is beginning (left hand side of the figure). EC, ectoderm ; N, nervous system ; R, dorsal groove; MW, medullary folds ; Mp, somatic mesoblast; Me, splanchnic mesoblast; Ck, noto- chord; End, intestinal endoderm ; D/i, lumen of gut; Lh, body cavity (pleuroperitoneal cavity); UW, protovertebra; D, yolk. their connection with accessory glands and external copulatory apparatuses determines the great variations in the structure of the generative organs which are most complicated in the Mammalia. In many Fishes
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