490 



STRUCTURE OF VERTEBRATA. 



ao- 



siv 



In Myxine and Bdellostoma it per- 

 sists in adult life, though apparently, 

 at least in part, in a degenerate con- 

 dition, and is said to be the functional 

 excretory organ of the little (de- 

 generate?) fish Fierasfer and some 

 other Bony Fishes. In most bony 

 Fishes, and in Amphibia, it is merely 

 a larval organ, but is then large and 

 important. In Elasmobranchs and 

 Amniota, except Crocodiles and 

 Turtles, it is from the first rudi- 

 mentary and functionless. 



The origin of the segmental or 

 pronephric duct is still undetermined. 

 It usually arises from the mesoblast, 

 in some cases growing backwards 

 directly from the rudiment of the 

 pronephros, while in others the sur- 

 rounding mesoblast takes an import- 

 ant part in its formation ; in Elasmo- 

 branchs, in Mammals, and in the 

 chick, a connection with the epiblast 

 has been described by various ob- 

 servers. Riickert is of opinion that 

 it originally arose by the fusion of 

 the outer ends of the pronephric 

 tubules, and that the occasional 

 connection with the ectoderm in- 

 dicates the position of former ex- 

 cretory pores (cf. A/nphloxiis}. 



FIG. 235. Development of Excre- 

 tory system of Vertebrate. --In 

 part after Boveri. 



In I. the primitive segments are not 

 separated off from the lateral plate, 

 and the pronephros (_/..) is seen aris- 

 ing from the lower part of the primitive 

 segment. In II. the pronephros is 

 completely separated off from the primi- 

 tive segment and lateral plate. In 

 III. the origin of the mesonephric 

 tubules is seen. They arise from the 

 upper part of the lateral plate, which 

 is now completely separated from the 

 primitive segment, and curving round 

 the pronephric duct come to open into 

 it. 



>i.c., nerve cord ; nch., notochord ; />., 

 pronephros; g:, gut; p.s., primitive 

 segment; Hies., mesonephric tubule; 

 pn.d., pronephric duct; fr.c., body 

 cavity; <?., aorta; siv., sub-intestinal 

 vein, with vessel to the aorta. 



