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AN ELEMENTARY TEXT-BOOK OF BIOLOGY. 



which they are connected. As the lungs develop they are 

 supplied by branches of the 6th afferent arteries, and, gradually 

 becoming functional, pour their blood into the auricle, which 

 becomes divided. At the same time the afferent and efferent 

 vessels become directly united to form continuous arches. The 

 internal gills meanwhile atrophy, and the 4th arches alone 

 remain connected to form the dorsal aorta. These changes are 

 best exhibited tabularly. 



The adult kidney is preceded by a rudimentary excretory organ, 

 the pronephros (head-kidney), consisting of a glandular con- 

 voluted tube, with three openings into the body-cavity. A 

 longitudinal pronephric dud ( = archinephric or segmental duct) 

 runs back from this and opens into the cloaca. The adult 

 kidney, more properly termed the Wolffian "body or mesonephros, 

 is developed as a number of tubules, at first comparatively simple, 

 which become connected with the body- cavity on the one hand, 

 and the pronephric duct on the other. This duct becomes the 

 urinogenital duct of the male and the ureter of the female. 



The ovary and spermary are developed as thickenings of the 

 peritoneum lining the body-cavity. The oviduct (Miillerian duct) 

 commences on the dorsal wall of the body-cavity as a groove 

 which is subsequently converted into a tube. 



The voluntary muscles are formed from the somatic layers of 

 the mesodermic somites. 



