PISCES 



187 



Mesoneph. tub. 

 'roneph. d. 



Body cavity 

 (splanchnocod 



outgrowth of the nephrotome in their case has to establish 

 an opening into the pronephric duct. A large number of 

 segmental tubules are formed in this way and successively 

 connected with the pronephric duct. This second set of 

 tubules constitutes the mesonephros. 



Each tubule is connected to the splanchnocoel by a nephro- 

 stome or peritoneal funnel, and the other end opens into the 

 pronephric duct. Between these two openings the tube 

 expands to form a Bowman's capsule, and the capsule is 

 indented by a glomerulus 

 and thus a Malpighian 

 body is produced. 



The segmental tu- 

 bules, to begin with, are 

 small and gradually gain 

 in size and functional 

 importance posteriorly. 

 In the female, moreover, 

 the first seven to nine 

 mesonephric tubules are 

 rudimentary. In the 

 male they persist to take 

 part in the formation of 

 the vasa efferentia. FlG 94 ._ Transverse section through 



In association with mesonephros of embryo of Acanthias. 



sex, moreover, a change 



takes place which converts the pronephric duct into male and 

 female gonoducts. In the female, one of the pronephric nephro- 

 stomes expands, while the rest are atrophied and disappear. 

 This persisting opening forms the inner opening of the oviduct 

 or Miillerian duct. Then further back the pronephric duct 

 where it is associated with mesonephric tubules is split off 

 longitudinally, and the longitudinal division goes on posteriorly 

 so as to remove the part of the original duct connected with 

 the tubules from the part communicating in front with the 

 persisting opening of the pronephros. The duct is therefore 

 divided into an oviduct and a mesonephric duct. 



In the male the two ducts are similarly divided, but the 

 anterior seven to nine mesonephric tubules are connected to 



