260 STRUCTURE OF THE VERTEBRATES 



eggs break from the ovaries they enter the ostia and pass into 

 the oviducts. The dogfish has internal fertilization and the eggs 

 remain in the oviducts until the embryos are capable of inde- 

 pendent life. This is true of most sharks. In the skates, rays, and 

 most teleosts the eggs are thrown into the water and fertilized 

 externally. 



The male dogfish undergoes more radical changes during 

 development. The testes remain in the primitive anterior posi- 

 tion, and the mesonephric tubules of the anterior (sexual) por- 

 tion of the kidney grow through the mesonephros and enter the 

 gonad. The Wolffian ducts, therefore, function as both urinary 

 and genital outlets. The papilla through which the united ducts 

 leave the body is called the urinogenital papilla. The IMlillerian 

 ducts degenerate, and disappear entirely except for a small, 

 vestigial mass of tissue. 



The system as described for the dogfish is typical of that 

 of the fish and amphibia. In the vertebrates above the Elasmo- 

 branchs the oviducts arise parallel to the Wolffian ducts, but 

 from the same anlage. The essential changes during develop- 

 ment are the same. In those fish which retain their eggs during 

 development (the ovo-viviparous condition) the male develops 

 specialized intromittent organs for placing the sperms in the 

 oviducts, so that internal fertilization may take place. A few 

 amphibia have internal fertilization but deposit their eggs in 

 an early stage of cleavage; but in this group the sperms are 

 usually deposited in capsules, and the female has a specialized 

 mechanism for taking up the sperm capsules. 



Amniotes. Amniote embryos develop oviducts and meso- 

 nephric ducts similar to those of the fish and amphibia. A 

 slightly generalized drawing of the undifferentiated embryonic 

 stage would apply equally to any vertebrate above the cyclo- 

 stomes. Amniote embryos add, however, the metanephric kid- 

 ney and the mesonephros gradually degenerates. In both sexes 

 the Wolffian ducts lose their excretory function. 



The female amniote loses the mesonephros and the meso- 

 nephric ducts, a small fragment of tissue remaining in the 

 mesentery as a vestigial body. The IMlillerian ducts remain as 

 the oviducts and undergo various modifications in correlation 



