70 ANATOMY OF AMPHIOXUS. 



These outgrowths are at first solid cell-proliferations of 

 the peritoneal epithelium, in the midst of which a lumen 

 is subsequently formed between the cells. As soon as 

 this occurs, the peritoneal thickenings represent hollow 

 diverticula of the ccelom, each communicating with the 

 latter by a single nephrostome (Fig. 33). 



The incipient tubules then grow outwards until they 

 reach the ectoderm with which, in the Selachians, they 

 become fused. This has been taken by Ruckert to indi- 

 cate that the tubules originally discharged the products 

 of excretion directly to the exterior by a series of indepen- 

 dent apertures at the points of fusion. (Cf. Fig. 34 A.) 5 

 The pronephric tubules next commence gradually to relin- 

 quish their coalescence with the ectoderm from before 

 backwards, retaining, however, for the present the connec- 

 tion behind (Fig. 34 B}. 



Meanwhile the distal ends of the successive tubules 

 undergo confluence (Fig. 34 B), and in this way the begin- 

 ning of a longitudinal duct is produced. This duct now 

 gradually splits itself off from the ectoderm, so that the 

 posterior connection with the latter is carried farther and 

 farther back until it reaches the region of the cloaca, when 

 it leaves the ectoderm and acquires an opening into the 

 cloaca (Fig. 34 C). Meanwhile, however, in the Sela- 

 chians, the pronephric tubules begin to undergo a retro- 

 gressive development and atrophy, as a consequence of 

 which the pronephros as a gland becomes aborted. 



In the same way, but at a much later stage, the remark- 

 able pronephric system of Ichthyophis becomes entirely 

 aborted. But the duct remains, and a new set of tubules 

 appear at the bases of the somites, which secondarily open 

 into it (Fig. 34 C). 



These new tubules are the mesonephric tubules, and, 



