5 io 



DEVELOPMENT OF ELASMOBRANCH FISHES. 



repeat them in this place. I conclude the chapter with a resume 

 of the development of the urinogenital organs in Elasmobranchs 

 from their first appearance to their permanent condition. 



Resume. The first trace of the urinary system makes its 

 appearance as a knob springing from the intermediate cell-mass 

 opposite the fifth proto vertebra (woodcut, fig. 5 A,/, d}. This 

 knob is the rudiment of the abdominal opening of the segmental 

 duct, and from it there grows backwards to the level of the anus 

 a solid column of cells, which constitutes the rudiment of the 

 segmental duct itself (woodcut, fig. 5 B, /. d]. The knob projects 



FIG. 5. 

 Two SECTIONS OF A PRISTIURTJS EMBRYO WITH THREE VISCERAL CLEFTS. 



A 



spn 



spn 



The sections illustrate the development of the segmental -duct (pd) or primitive 

 duct of the kidneys. In A (the anterior of the two sections) this appears as a solid 

 knob (pd) projecting towards the epiblast. In B is seen a section of the column 

 which has grown backwards from the knob in A. 



spn. rudiment of a spinal nerve; me. medullary canal; ch. notochord ; X. string 

 of cells below the notochord; mp. muscle-plate; mp'. specially developed portion of 

 muscle-plate ; ao. dorsal aorta ; pd. segmental duct ; so. somatopleure ; sp. splanchno- 

 pleure; pp. pleuroperitoneal or body-cavity; ep. epiblast; al. alimentary canal. 



towards the epiblast, and the column connected with it lies 

 between the mesoblast and epiblast. The knob and column do 

 not long remain solid, but the former acquires an opening into 

 the body-cavity continuous with a lumen, which makes its 

 appearance in the latter. 



While the lumen is gradually pushing its way backwards 

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



