PETROMYZON 33 



noted that the tubules which form it are of mesodermal origin, 

 coming from the ccelomic epithelium, and are sharply to be 

 distinguished from nephridia, which are ectodermal in origin. 



Originally these coelomostomes must have served to free the 

 genital products (as indeed the spermatozoa are freed in all 

 higher forms), and excretion also took place through them from 

 the coelom to the exterior. Then the excretory products were 

 brought to the tubules by blood-vessels and the coelom lost its 

 excretory function. The tubules lose their connexion with the 

 splanchnocoel. In Petromyzon the coelomostomes were never 

 open. In other forms, the coelomostomes may persist. In 

 Myxine, a close relative of Petromyzon, the Malpighian 

 corpuscles retain their segmental arrangement, but in all other 

 forms they become increased in number, and the segmental 

 correspondence is lost. 



The tubules arise in two sets. First an anterior lot, opening 

 into the pericardium, form the pronephros, and their duct (the 

 pronephric duct) grows right back to the papilla. The 

 pronephros nearly disappears in the adult Petromyzon, and is 

 replaced in function by an identical but more posterior set of 

 tubules which form the mesonephros. The mesonephric 

 tubules grow into the pronephric duct which they find ready- 

 made for them, and which becomes known as the mesonephric 

 duct. The mesonephric tubules develop later than the 

 pronephric tubules, but are of essentially the same nature. 

 This is illustrated in Bdellostoma, another relative of Petro- 

 myzon, in which a continuous series of tubules arises, the more 

 anterior of which become the pronephros, and the posterior 



venule to the posterior cardinal vein (pc). C, condition in most young 

 chordates, in which an anterior set of pronephric tubules (pt) gives rise to 

 a pronephric duct (pd) before the more posterior mesonephric tubules 

 (dmt) are properly formed, pf, pronephric funnels ; mf, mesonephric 

 funnels. D, condition in older chordates, and retained throughout life in 

 Myxine. The pronephros has degenerated, and the mesonephric tubules 

 (mi) have joined the pronephric duct which now bears the name mesonephric 

 duct (md). The latter is no longer continuous with the anterior portion of 

 the pronephric duct. The capsules of the mesonephros lose their connexion 

 with the splanchnocoel, but they retain their simple segmental arrangement. 

 E, condition in Petromyzon. The pronephros has degenerated, and the 

 number of capsules in the mesonephros has been increased by the formation 

 of secondary Bowman's capsules (sBc). 



D 



