-Mullerian duct- 



^nephric duct 



%. 



B 



17 mm 



C 25.3 mm 

 MALE 



D 35 mm 



FEMALE 



Figure 10-25. Two stages of kidney development in Prisfjurus (the 

 sawfish). A, progressive fusion of pronephric funnels to form ostium of 

 Mijllerian duct in the early stage; B, C, D, later stage of male and 

 female. (After van den Broek et al., 1938) 



uli to a marginal canal and independent extensions of the 

 central net of the testis, through the connective tissue bands 

 of the mesorchium to the kidney and into the marginal 

 canal. 



The nephric duct of the male also is modified in forming 

 the ductus, or vas, deferens. Behind the epididymis, 10 to 

 15 segments are modified into the Leydig's gland. Behind 

 this, 10 or 11 segments remain as the definitive kidney. 



Ho/ocepha/on The kidney of the holocephalan resembles 

 that of the shark (Figure 1 0-49). It is an elongate band, nar- 

 row anteriorly, somewhat expanded posteriorly. The most 



anterior part in the male is an epididymis; behind this is a 

 Leydig's gland followed by the functional kidney. Since 

 Leydig's gland and the kidney cannot be differentiated 

 macroscopically, the term Leydig's gland is applied loosely 

 to the narrow anterior part. On the basis of collecting tu- 

 bules, there are six to eight segments in the Leydig's gland, 

 which is excetory in the young male but becomes aglomerular 

 and secretory in the adult. The most anterior lobules of the 

 functional kidney drain into the much convoluted vas defe- 

 rens which then expands into a thick-walled ampulla for 

 most of the length of the functional kidney. The greater 

 part of the functional kidney, eight or nine lobules, drains 

 through a series of ureters. Some of these ureters have com- 

 plexly looped sections and the first six or so enter the am- 

 pulla dorsally. The last two enter the urinary sinus behind the 

 ampulla. The nephric ducts fuse posteriorly to form a uro- 

 genital sinus. In the mature male there is a small glandular 

 mass, the urogenital gland, projecting forward from the uro- 

 genital sinus just before that sinus opens from the body. There 

 are no peritoneal funnels in the kidney but abdominal pores 

 are present. There are vestigial Mullerian ducts in the male. 



The kidney of the female is like that of the young male 

 except that the most anterior part degenerates (there is no 

 epididymis). The extreme posterior ends of the kidneys may 

 fuse, but they are essentially separate bands between which 

 the interrenal gland lies. The nephric duct and four of five 

 ureters join and empty into a thick-walled (muscular) 

 urinary sinus which extends forward as a bladder. The 

 Miillerian ducts are well developed and posteriorly open 

 into a slightly developed urogenital sinus (Figure 10-50). 



The development of the kidney of this group has not been 

 described. It is assumed that in the young female the uro- 

 genital sinus opens into a cloaca. The urogenital sinus has 

 a pouch-like outgrowth. As this urogenital sinus everts, the 

 Mijllerian ducts, the excretory pore, and a more anterior 

 "seminal receptacle" come to open almost separately. 



Genera/ observofions There is a close resemblance be- 

 tween the shark and the holocephalan in terms of the excre- 

 tory system, at least in the intricately looped and knotted 

 vas deferens. One item of difference lies in the fact that the 

 urogenital system opens behind the anus. Another item is 

 the forward-projecting "seminal receptacle" of the female 

 holocephalan which is suggestive of the bladder in higher 

 forms and the rectal gland of the shark; that is, it is a 

 rudimentary allantois. Both shark and chimaerid have 

 abdominal pores opening posterolateral to the anal or 

 cloacal opening. 



The development of the shark kidney differs markedly 

 from that of the other fishes and tetrapods in that the tu- 

 bules of all segments, not just the pronephric region, arise 

 directly or indirectly from funnels. The mesonephric tubules 

 arise as several generations of evaginations from a nephro- 

 coel (lateral vesicle). The collecting ducts arise as out- 

 growths from the nephric duct in the mesonephric area as 



312 



THE UROGENITAL SYSTEM 



