692 



ELASMOBRANCHII. 



.o 



-SJJ. V 



backwards round the inner and dorsal side of the segmental 

 duct, near which it at first ends blindly. 



One segmental tube makes its 

 appearance for each somite (fig. 265), 

 commencing with that immediately 

 behind the abdominal opening of the 

 segmental duct, the last tube being 

 situated a few segments behind the 

 anus. Soon after their formation 

 the blind ends of the segmental tubes 

 come in contact with, and open into 

 the segmental duct, and each of them 

 becomes divided into four parts. 

 These are (i) a section carrying the 

 peritoneal opening, known as the 

 peritoneal funnel, (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 

 primitive position between the epi- 

 blast and mesoblast, and assumes a 

 position close to the epithelium lining 

 the body cavity (fig. 380, sd}. The 

 general features of the excretory 

 organs at this period are diagrammatically represented in the 

 woodcut (fig. 387). In this fig. fid is the segmental duct and 

 o its abdominal opening ; s.t points to the segmental tubes, 

 the finer details of whose structure are not represented in the 

 diagram. The mesonephros thus forms at this period an elon- 

 gated gland composed of a series of isolated coiled tubes, one 

 extremity of each of which opens into the body cavity, and the 

 other into the segmental duct, which forms the only duct of the 

 system, and communicates at its front end with the body cavity, 

 and behind with the cloaca. 



FIG. 386. SECTION THROUGH 

 THE TRUNK OF A SCYLLIUM EM- 

 BRYO SLIGHTLY YOUNGER THAN 



28 F. 



sp.c. spinal canal; W. while 

 matter of spinal cord ; pr. poste- 

 rior nerve-roots ; ch. notochord ; 

 x. sub-notochordal rod ; ao. aorta ; 

 mp. muscle-plate ; mp' . inner layer 

 of muscle-plate already converted 

 into muscles ; I'r. rudiment of 

 vertebral body ; st. segmental 

 tube; sd. segmental duct; sp.v. 

 spiral valve ; v. subintestinal vein ; 

 P.O. primitive generative cells. 



