808 COMPARATIVE ANATOMY 



grow outward until they reach just below the ectoderm when they bend 

 toward the caudal end of the body. Here the more cephalic tubules fuse 

 with those behind and it is at this meeting place of the tube that the 

 pronephric duct, sometimes called the archinephric duct, grows back- 

 ward immediately beneath the ectoderm. This backward growth con- 

 tinues until the caudal end of the metacoele has been reached. It is here 

 that the pronephric duct fuses with the caudal end of the digestive tract 

 and empties into the cloaca, as in the frog, where it meets with the 

 ectoderm close to the anal opening. In either case an opening then 

 breaks through so that the contents of the duct can be expelled. 



The question is often asked as to whether the ducts thus formed 

 are of mesothelial origin or whether the ectoderm contributes a share. 

 From present evidence it is assumed that the ectoderm has no share 

 in their formation. 



"The pronephros is functional for a time in the embryos of some 

 lower vertebrates ; in other groups it is a rudimentary and transitory 

 structure save for its participation in the oviducts and the ostium tubae 

 abdominale. When functional, it takes the nitrogenous waste from 

 the body cavity, while its filtering apparatus consists either of separate 

 glomeruli (one for each tubule) or the glomeruli of the separate somites 

 may run together, forming a glomus. These glomeruli or the glomus 

 of the pronephros do not project into a Bowman's capsule, but lie imme- 

 diately above the dorsal wall of the coelom, between the mesentery and 

 the nephrostomes, pushing the epithelium before them. Later, as in the 

 caecilians, they and the nephrostomes may be enclosed in a cavity cut 

 off from the coelom, so that the whole resembles a renal corpuscle, but is 

 different in origin. In either case the exuding fluid passes into the meta- 

 coele, from which it is drawn by the cilia of the nephrostomes and passed 

 into the tubules. 



"The blood is brought to the glomus or glomeruli by short seg- 

 mental arteries arising from the dorsal aorta and, after passing through 

 the capillaries, it is carried away by the postcardinal veins of the cor- 

 responding side to the heart, these veins keeping pace in their backward 

 development with the development of the nephridial tubules." 



In all vertebrate adults, with the possible exception of Bdellostoma 

 (Fig. 366), the pronephros has been replaced by the mesonephros and 

 later still in the amniotes by the metanephros. In the cyclostomes and 

 a few teleosts the pronephros, however, persists. 



THE MESONEPHROS. 



The mesonephros, also called the Wolffian body, is formed by a 

 series of mesonephric tubules which are developed after the pronephros 

 and its ducts are completely formed. The mesonephric tubules grow 

 out from the nephrotomes behind those which form the pronephros. The 

 tubules extend toward each side of the animal until they meet and fuse 

 with the pronephric duct. This duct is then the excretory canal for the 



