EXCRETORY ORGANS. 



731 



In the majority of the lower Vertebrata the mesonephric 

 tubes have at first a segmental arrangement, and this is no 

 doubt the primitive condition. The coexistence of two, three, or 

 more of them in a single segment in Amphibia, Aves and 

 Mammalia has recently been shewn, by an interesting discovery 

 of Eisig, to have a parallel amongst Chaetopods, in the co- 

 existence of several segmental organs in a single segment in 

 some of the Capitellidae. 



In connection with the segmental features of the meso- 

 nephros it is perhaps worth recalling the fact that in Elasmo- 

 branchii as well as other types there are traces of segmental 

 tubes in some of the postanal segments. In the case of all the 

 segmental tubes a Malpighian body becomes established close 

 to the extremity of the tube adjoining the peritoneal opening, or 

 in an homologous position in tubes without such an opening. 

 The opposite extremity of the tube always becomes attached to 

 the segmental duct. 



In many of the segments of the mesonephros, especially in 

 the hinder ones, secondary and tertiary tubes become developed 

 in certain types, which join the collecting canals of the primary 

 tubes, and are provided, like the primary tubes, with Malpighian 

 bodies at their blind extremities. 



There can it appears to me be little or no doubt that the 

 secondary tubes in the different types are homodynamous if not 

 homologous. Under these circumstances it is surprising to find 

 in what different ways they take their origin. In Elasmo- 

 branchii a bud sprouts out from the Malpighian body of one 

 segment, and joins the collecting tube of the preceding segment, 

 and subsequently, becoming detached from the Malpighian body 

 from which it sprouted, forms a fresh secondary Malpighian 

 body at its blind extremity. Thus the secondary tubes of one 

 segment are formed as buds from the segment behind. In 

 Amphibia (Salamandra) and Aves the secondary tubes develop 

 independently in the mesoblast. These great differences in 

 development are important in reference to the homology of 

 the metanephros or permanent kidney, which is discussed 

 below. 



Before leaving the mesonephros it may be worth while putting forward 

 some hypothetical suggestions as to its origin and relation to the pro- 



