THE ELASMOBRANCH FISHES 



299 



BOWMAN'S CAPSULES 



According to Borcea (1906) a Bowman's capsule is i)roduced from a part of 

 the median vesicle in the following manner (Acanthias). The median vesicle 

 (fig. 258b-c) divides into two parts, a median part connected with the seg- 

 mental duct (s.d.) and an outer (deeper) part continued by the renal tubule 

 (r.f.). The outer part is the first to give rise to a Bowman's capsule (fig. 258c) 

 and this is accomplished by the loose cells from the lining forming over the 

 aperture caused by the separation of the median vesicle. Into this the knot of 



u 



kj 



nph 



pr.d.. 



A 



B 



C 



D 



Fig. 258. Diagrams A to E to show stages in the development of Bowman's capsules, 

 Acanthias. (From Borcea.) 



c.t., collecting tubule; m.v., median vesicle; nph., nephrostome; pr.d., pronephrotic duct; 

 r.t., renal tubule; s.d., segmental duct; I, II, and III, primary, secondary, and tertiary renal 

 corpuscles. 



blood vessels (the glomerulus) pushes. The body segment is thus provided at 

 first with a single primary (I) renal corpuscle on each side, and a true metam- 

 erism obtains in the kidney tissue. As development progresses, however, 

 secondary and tertiary Bowman's capsules are formed. Some of these pass 

 over into the adjacent segments and soon destroy the primitive metamerism. 

 The formation of the secondary Bowman's capsules and their connection 

 with the pronephrotic duct takes place as follows: from that part of the 

 median vesicle of the segmental canal which remains after the formation of a 

 primary, that is, from the superior and inferior vesicles, other Bowman's 

 capsules (secondary and tertiary) arise. The secondary capsules are formed 

 from the median part of each superior and inferior vesicle and then extend in 

 the shape of a gourd toward the collecting tubule (see fig. 258d, //) . It will be 

 further noted that enlargements arise on each collecting tubule (c.t.) and 

 that these send out processes which meet and fuse with the tips of the gourd- 

 like structure, the superior processes fusing with the terminus of the sec- 

 ondary Bowman's capsule derived from the inferior vesicle. Upon the fusion 

 of this terminus of the gourd with the process of the collecting tubule and the 

 breaking through of the connection between them, a secondary urinary tubule 

 results (//, fig. 258e). 



