298 



THE ELASMOBRANCH FISHES 



leeted in tlie median vesicles at the end of the segmental duct (see fig. 254a, 



m.v.) and there appeared as large colored patches. In no specimen did the ink 



or carmine which entered the nephrostome gain access to the tissue of the 



kidney. 



DEVELOPMENT OE NEPHKOSTOME AND 



SEGMENTAL DUCT 



A section through the body cavity of an embryo of Heterodontus francisci 

 (fig. 257) cuts through the nephrostome or funnel (nph.). If traced farther 



back, it would be found that this funnel 

 by means of the segmental duct joins the 

 pronephrotic duct (pr.d.). At first the 

 most anterior of such funnels may pro- 

 vide a passageway from the body cavity 

 to the pronephrotic duct, but later the 

 most anterior of these fuse into a single 

 enlarged funnel. Those segmental funnels 

 arising back of this have segmental ducts 

 (s.d., fig. 258) which join the pronephrotic 

 duct only secondarily. 



In the segments farther back, however, 

 each segmental duct of Acanthias grows 

 from its funnel laterally, enlarging into a 

 median vesicle (m.v., fig. 258a) . From this 

 it may even continue inward to join an 

 outgrowth from the pronephrotic duct 

 (pr.d.). The following structures are 

 found in order from the funnel or nephro- 

 stome to the pronephrotic duct: (1) the 

 nephrostome (nph.); (2) its segmental 

 duct (s.d.); (3) a median vesicle (m.v.) 

 which supplies the tissue for Bowman's 

 capsules; (4) the tube leaving the capsule which becomes the renal tubule 

 (r.t.); and (5) the primitive collecting tubule (c.t.) which according to 

 Borcea (1906) buds off from the pronephrotic duct and later lengthens out 

 into the collecting tubule of the adult. 



At this point in its development, connection is made from the nephrostome 

 to the pronephrotic duct (ScylUnm, fig. 256), and it is not impossible that 

 while this temporary connection lasts nitrogenous waste may pass from the 

 body cavity through the pronephrotic (now the Wolffian) duct and out 

 through the cloaca. But this connection is early lost even in Scyllium. Such 

 connection between the coelom and the kidney tissue is never actually present 

 in Acanthias, for in it the tube early fragments at the median vesicle before 

 its terminal part, the renal tubule, has reached the pronephrotic (Wolffian) 

 duct. It is from the fragmenting tissue of the median mass that Bowman's 

 capsules are formed. 



Fig. 257. Development of nephrostome, 

 Heterodontus francisci. (H. M. William- 

 son, orig.) 



np/i., nephrostome; of7., oviduct ; pr.d., 

 pronephrotic duct. 



