THE URINOGENITAL ORGANS OF VERTEBRATES. 137 



spn 



FlG. I. TWO SECTIONS OF A PRISTIURUS EMBRYO WITH THREE VISCERAT. 



CLEFTS. 



The sections are to shew the development of the segmental duct (pd) or primi- 

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

 solid knob 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 ; ;;//'. specially developed 

 portion of muscle-plate ; ao. dorsal aorta ; pd. segmental duct. so. somatopleura ; 

 sp. splanchnopleura ; //. pleuroperitoneal or body cavity; cp. epiblast; al. ali- 

 mentary canal. 



duct closed behind, but communicating in front by a wide 

 opening with the pleuroperitoneal cavity. 



Before these changes are accomplished, a series of solid 1 

 outgrowths of elements of the 'intermediate cell- mass' appear 

 at the uppermost corner of the body-cavity. These soon be- 

 come hollow and appear as involutions from the body-cavity, 

 curling round the inner and dorsal side of the previously formed 

 duct. 



One involution of this kind makes its appearance for each 

 protovertebra, and the first belongs to the protovertebra im- 

 mediately behind the anterior end of the duct whose develop- 

 ment has just been described. In Pristiurus there are in all 

 29 of these at this period. The last two or three arise from 

 that portion of the body-cavity, which at this stage still exists 

 behind the anus. The first-formed duct and the subsequent 

 involutions are the rudiments of the whole of the urinary system. 



1 These outgrowths are at first solid in both Pristiurus, Scyllium and Torpedo, but 

 in Torpedo attain a considerable length before a lumen appears in them. 



