280 



RESUME OF UROGENITAL SYSTEM. 



dered necessary a few corrections in my facts, I still adhere 

 in their entirety to the views expressed in^that paper, and feel 

 it unnecessary to 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 protovertebra (woodcut, fig. 5 A p.d). This 

 knob is the rudiment of the abdominal opening of the seg- 

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

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

 of tlie segmental duct itself (woodcut, fig. oB pxl). The knob 

 projects 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. 



Fig. 5. 



Two Sections of a Pristiurus Embryo with three visceral clefts. 



A 

 spn 



The sections illustrate the development of the segmental duct {pel. ) or primi- 

 tive duct of the kidneys. In A (the anterior of the two sections) 

 this appears as a solid knob {pel) 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; mc. medullary canal; ch. notochord; 

 X. string of ceUs below the notochord; mp. muscle-plate; mp'. specially 

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

 so. somatopleure ; sp. splanchnopleure ; pp. pleuroperitoneal or body- 

 cavity; ep. epiblast; al. ahmentary canal. 



