250 EMBRYOLOGY OF THE LOWER VERTEBRATES ch. 



geophis though in the case of the Elasmobranchs the details seem to 

 be more obscure and the descriptions are conflicting. 



The male Elasmobranch is an excellent example of a Vertebrate 

 in which the nephridial system is responsible for the function of 

 conveying to the exterior both the renal excretory materials and the 

 reproductive cells and we find a well-marked tendency to separate 



the routes of those two products to- 

 wards the exterior. This separation 

 is brought about by the shifting back- 

 wards of the openings of the collecting- 

 tubes of the posterior, purely renal, 

 part of the opisthonephros, so that 

 instead of being spaced out along the 

 course of the duct they come to be 

 coincident with its opening into the 

 urinogenital sinus. This backward 

 shifting is most pronounced, and it 

 also makes its appearance earliest in 

 ontogeny, in the most anterior of the 

 tubules in question. It is accompanied 

 by a fusion together of the terminal 

 parts of the collecting -tubes, into a 

 continuous ridge-like projection of the 

 dorsal wall of the duct, in which the 

 individual lumina are for a time greatly 

 reduced or even completely obliterated. 

 Eventually, as a rule, the ridge splits 

 up and the terminal parts of the 

 collecting - tubes regain their indi- 

 viduality — forming a group of distinct 

 tubes, varying in number in different 

 forms from about 4 (Spinax) to about 

 15 (Acanthias), and converging so as 

 to open close together into the urino- 

 genital sinus. In some cases the split- 

 ting apart is not complete and more or 

 fewer of the tubes may be united to- 

 gether to form a longitudinal " ureter." 

 The mode of origin of the Mal- 

 pighian body — the definitive condi- 

 tion of the nephrotome — from which 

 each opisthonephric tubule leads has already been indicated. It 

 is for a time rounded in form (Fig. 135) but eventually one portion 

 of its wall — varying greatly in position — comes to bulge inwards to 

 form the glomerulus containing a loop of blood-vessel. 



The peritoneal canal during development lengthens out consider- 

 ably (Fig. 135, D) and becomes narrower. This narrowing is most 

 marked in the posterior third of the opisthonephros and in this we 



Pig. 135. — Illustrating the later 

 development of a segmental unit 

 of the opisthonephros in male 

 Pristiurus. The figure is in each 

 case a view from the mesial side. 

 (After C. Eabl, 1896.) 



A, 15th unit of 17 mm. embryo ; B, 15th 

 unit of 22-5 mm. ; C, 15th unit of 25-3 

 mm. ; D, 25th unit from same embryo as 

 C. m.li, Malpighian body ; p.c, peritoneal 

 canal ; p.f, peritoneal funnel ; t, tubule. 



