EXCRETORY ORGANS. 709 



peritoneal epithelium, and then develops a funnel-shaped opening into the 

 body cavity, which subsequently becomes ciliated. In this way the peritoneal 

 funnels which are present in the adult are established. 



The median and lateral sections of the segmental tubes become highly 

 convoluted, and the separate tubes soon come into such close proximity that 

 their primitive distinctness is lost. 



The first fully developed segmental tube is formed in Salamandra macu- 

 lata in about the sixth myotome behind the pronephros. But in the region 

 between the two structures rudimentary segmental tubes are developed. 



The number of primary segmental tubes in the separate myotomes of 

 Salamandra is as follows : 



In the 6th myotome (i.e. the first with a true 



segmental tube) 12 segmental tubes 



yth roth myotome 23 



IIth ... 34 



I2th 3 4 or 4 5 



I3th y> 45 



1 3th i6th 56 



It thus appears that the segmental tubes are not only more numerous than 

 the myotomes, but that the number in each myotome increases from before 

 backwards. In the case of Salamandra there are formed in the region of 

 the posterior (10 16) myotomes secondary, tertiary, etc. segmental tubes out 

 of independent solid cords, which arise in the mesoblast dorsally to the tubes 

 already established. 



The secondary segmental tubes appear to develop out of these cords 

 exactly in the same way as the primary ones, except that they do not join the 

 segmental duct directly, but unite with the primary segmental tubes shortly 

 before the junction of the latter with the segmental duct. In this way com- 

 pound segmental tubes are established with a common collecting tube, but 

 with numerous Malpighian bodies and ciliated peritoneal openings. The 

 difference in the mode of origin of these compound tubes and of those in 

 Elasmobranchii is very striking. 



The later stages in the development of the segmental tubes have not been 

 studied in the other Amphibian types. 



In Cceciliidas the earliest stages are not known, but the tubes present in 

 the adult (Spengel) a truly segmental arrangement, and in the young each of 

 them is single, and provided with only a single peritoneal funnel. In the 

 adult however many of the segmental organs become compound, and may 

 have as many as twenty funnels, etc. Both simple and compound segmental 

 tubes occur in all parts of the mesonephros, and are arranged in no definite 

 order. 



In the Anura (Spengel) all the segmental tubes are compound, and an 

 enormous number of peritoneal funnels are present on the ventral surface, 

 but it has not yet been definitely determined into what part of the segmental 

 tubes they open. 



