570 VEKTEBRATE LIFE AND ORGANIZATION 



the distal end of the vasa deferentia; a prostate gland surrounding the 

 urethra at the point of entrance of the vasa deferentia; and a pair of 

 Cowper's glands located more distally along the urethra. 



Female Vertebrates. Eggs are removed from the coelom in most 

 female vertebrates by a pair of oviducts, but the oviducts are modified 

 for various modes of reproduction. Lower vertebrates reproduce in the 

 water. Most are oviparous, fertilization is external, and the eggs de- 

 velop into larvae that can care for themselves. In the frog (Fig. 21.14), 

 each oviduct is a simple tube that extends from the anterior end of 

 the coelom to the cloaca. The oviducts may contain glandular cells that 

 secrete layers of jelly about the eggs, and their lower ends may be 

 expanded for temporary storage of the eggs, but they are not other- 

 wise specialized. 



Fertilization is internal in vertebrates that reproduce on the land, 

 and the free larval stage has been replaced by the evolution of a 

 cleidoic egg. Most reptiles and all birds are oviparous and the eggs 

 develop externally. The oviducal glands, which secrete the albumm 

 and a shell around the egg, are more numerous in the oviducts of 

 reptiles than in those of amphibians and most fishes, but in other 

 respects the oviducts of reptiles have not changed greatly. Birds have 

 lost the right oviduct along with the right ovary, but the remaining left 

 oviduct is essentially similar to the reptilian oviduct. 



Most mammals and a few fishes and reptiles have become vivip- 

 arous; they retain the fertilized egg within the reproductive tract until 

 embryonic development is complete. The oviducts are modified ac- 

 cordingly. In the human female (Figs. 28.5 and 28.6), the ostium lies 

 adjacent to the ovary and may even partially surround it. When ovula- 

 tion occurs, the discharged eggs are close enough to the ostium to be 

 easily carried into it by ciliary currents. The anterior portion of each 

 oviduct is a narrow tube known as the Fallopian tube, and eggs are 

 carried down it by ciliary action and muscular contractions. The re- 

 mainder of the primitive oviducts have fused with each other to form 

 a thick-walled, muscular uterus and part of the vagina. The terminal 

 portions of the vagina and urethra develop from a further subdivision 

 of the ventral part of the cloaca. The vagina is a tube specialized for 

 the reception of the penis. It is separated from the main body of the 

 uterus, in which the embryo develops, by a sphincter-like neck of 

 the uterus known as the cervix. The orifices of the vagina and urethra 

 are flanked by paired folds of skin, the labia minora and labia majora. 

 A small bundle of sensitive erectile tissue, the clitoris, lies just in front 

 of the labia minora. Structures comparable to these are present in the 

 sexually indifferent stage of the embryo, and develop into more con- 

 spicuous organs in the male. The labia majora are comparable to the 

 scrotum; the labia minora and clitoris, to the penis. A pair of glands, 

 homologous to Cowper's glands in the male, discharge a mucous secre- 

 tion near the orifice of the vagina. A fold of skin, the hymen, partially 

 occludes the opening of the vagina, but is usually ruptured during 

 the first intercourse. 



