156 THE URINOGENITAL ORGANS OF VERTEBRATES. 



This arises in the same way as in Selachians. A series of 

 involutions from the body-cavity are developed ; these soon form 

 convoluted tubes, which become branched and interlaced with 

 one another, and also unite with the primitive duct of the 

 kidneys. Owing to the branching and interlacing of the primi- 

 tive segmental tubes, the kidney is not divided into distinct 

 segments in the same way as with the Selachians. The mode 

 of development of these segmental tubes was discovered by 

 Gotte. Their openings are ciliated, and, as Spengel (loc. cit.} and 

 Meyer (Joe. cit.} have independently discovered, persist in most 

 adult Amphibians. As both these investigators have pointed 

 out, the segmental openings are in the adult kidneys of most 

 Amphibians far more numerous than the vertebral segments to 

 which they appertain. This is due to secondary changes, and is 

 not to be looked upon as the primitive state of things. At this 

 stage the Amphibian kidneys are nearly in the same condition 

 as the Selachian, in the stage represented in Fig. 2. In both 

 there is the segmental duct of the kidneys, which is open in 

 front, communicates with the cloaca behind, and receives the 

 whole secretion from the kidneys. The parallelism between the 

 two is closely adhered to in the subsequent modifications of the 

 Amphibian kidney, but the changes are not completed so far in 

 Amphibians as in Selachians. The segmental duct of the 

 Amphibian kidney becomes, as in Selachians, split into a Miil- 

 lerian duct or oviduct, and a Wolffian duct or duct for the 

 kidney. 



The following points about this are noteworthy : 



(1) The separation of the two ducts is never completed, so 

 that they are united together behind, and for a short distance, 

 blend and form a common duct ; the ducts of the two sides so 

 formed also unite before opening to the exterior. 



(2) The separation of the two ducts does not occur in the 

 form of a simple splitting, as in Selachians. But the efferent 

 ductules from the kidney gradually alter their points of en- 

 trance into the primitive duct. Their points of entrance become 

 carried backwards further and further, and since this process 

 affects the anterior ducts proportionally more than the posterior, 

 the efferent ducts finally all meet and form a common duct 

 which unites with the Mullerian duct near its posterior ex- 



