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MANUAL OF ELEMENTARY ZOOLOGY 



disappears altogether. The fourth branchial arch 

 becomes the pulmo-cutaneous. It loses its connection 

 with the aorta save for a vestige in the form of a ligament 

 in most adults, but sometimes, as always in the newt, an 

 open connection persists, and is known as the ductus 

 arteriosus. 1 We have seen (p. 449) that it is present also 

 during the development of the rabbit, where a vestige 

 remains in the adult. 



Fig. 382. — Diagrams of the development of the excretory system of the 

 frog. — From Bourne. 



A, The system of a tadpole about 12 mm. long, showing the pronephros and origin 

 of the mesonephric tubules ; B, the system at the end of metamorphosis. The 

 broken line represents approximately the position of the strip of peritoneal 

 epithelium which gives rise to the oviduct. 



c/., Cloaca ; d.ao., dorsal aorta ; f.b., fat body ; gl. % glomerulus l£".r. t genital ridge ; 

 mes., mesoneDhros ; ms.t., mesonephric tubules ; od. y oviduct ; pzj/T, position of 

 oviducal opening ; pn.f.^ pronephric funnels ; pnp.^ pronephros ; sg. t segmental 

 duct. 



The first rudiment of the excretory system appears some 

 time before hatching as a longitudinal thicken- 

 exoretory and i n g of the somatic layer of mesoblast on each 

 organs." side of the body, at the front end of the peri- 

 toneal cavity, immediately below the myotonies. 

 The front part of this thickening becomes converted into 

 the head kidney or pronephros (p. 354), which consists of 

 three twisted tubules, each opening by a funnel into the 



1 The terms ductus Botalli and ductus arteriosus are often used as 

 equivalent to one another, each being applied to both of the little 

 vessels which complete the carotid and pulmonary arches. 



