RESUME OF URINOGENITAL SYSTEM. 511 



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) (i) 

 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 primitive 

 position between the epiblast and mesoblast, and assumes a 

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

 general features of the excretory organs at this period are dia- 

 grammatically represented on the woodcut, fig. 6. In this fig. 



FIG. 6. 



DIAGRAM OF THE PRIMITIVE CONDITION OF THE KIDNEY IN AN ELASMOBRANCH 



EMBRYO. 



o 



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

 into the cloaca ; x. line along which the division appears which separates the 

 segmental duct into the Wolman duct above and the Mullerian duct below; st. 

 segmental tubes. They open at one end into the body-cavity, and at the other into 

 the segmental duct. 



p.d 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 kidneys thus form at this 

 period an unbroken gland composed of a series of isolated coiled 



