DEVELOPMENT OF ELASMO BRANCH FISHES. 281 



Whife the lumen is gradually pushing its way backwards 

 along the solid rudiment of the segmental duct, the first traces 

 of the segmental tubes, or proper excretory organs, make their 

 appearance in the form of solid outgrowths of the intermediate 

 cell- mass, which soon become hollow and open into the body- 

 cavity. Their blind ends curl obliquely backwards round the 

 inner and dorsal side of the segmental duct. One segmental 

 tube makes its appearance for each protovertebra, commencing 

 with that immediately behind the abdominal opening of the 

 segmental duct, the last tube being situated a short way behind 

 the anus. Soon after their formation the blind ends of the 

 segmental tubes open into the segmental duct, and each of 

 them becomes divided into four parts. These are (woodcut 7) 

 (1) a section carrying the abdominal opening or segmental tube 

 proper, (2) a dilated vesicle into which this opens, (3) a coiled 

 tubulus proceeding from (2) and terminating in (4), a wider portion 

 opening into the segmental duct. At the same time, or shortly 

 before this, each segmental duct unites with and opens into 

 one of the horns of the cloaca, and also retires from its primi- 

 tive position between the epiblast and mesoblast, and assumes 

 a position close to the epithelium lining the body-cavity. 



Fig. 6. 

 Diagram of the peimitive condition of the Kidney in asi Elasiiobranch 



EilBEYO. 



pd. Regmental duct. It opens at o into the body-cavity and at its other 

 extremity into the cloaca ; x. line along wliich the division appears which 

 separates the segmental duct into the Wolffian duct above and the Miilleriau 

 duct below; st. segmental tubes. They open at one end into the body- 

 cavity, and at the other into the segmental duct. 



The general features of the excretory organs at this period 

 are diagrammatically represented on the woodcut, fig. 6. In 

 this fig. 2^.d is the segmental duct and o its abdominal opening. 

 s.t points to the segmental tubes, the finer details of whose 

 ^- 10 



