GENERATIVE ORGANS. 



127 



usually unite to form an unpaired terminal section the urethra, 

 which, in Teleosteans only, opens behind the anus; very often it opens 

 into the cloaca, and in Mammals almost always unites with the 

 terminal parts of the genital ducts to form a common urogenital canal. 

 A vesicular reservoir the urinary bladder 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- Q 

 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 v 

 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 

 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 and Amphibia copulation is confined to an 

 external union of the two sexes, and the eggs are fertilized in 

 the water. Most Fishes, many Amphibia and Reptiles, and all Birds 

 lay their eggs. All the Mammalia are viviparous and their small 

 ova undergo embryonic development in the female generative ducts. 



End 



FIG. 581. Transverse section through a young embryo of 

 Triton taeniatus (after O. Hertwig). a, First appearance of 

 the medullary folds and formation of the notochord. 

 b, Closing of the medullary groove. The notochord is 

 completely separated off from the entoderm. The constric- 

 tion of the mesoderm into the protovertebrse is beginning 

 (left hand side of the figure). EC, ectoderm ; N, nervous 

 system ; B, dorsal groove ; MW, medullary folds ; Mp, 

 somatic mesoblast ; Mv, splanchnic mesoblast ; Ch, noto- 

 chord; End, intestinal endoderm; Dh, lumen of gut; Lh, 

 body cavity (pleuroperitoneal cavity); UW, protovertebra; 

 D, yolk. 



