Haddon — On the Epiblastic Origin of the Segmental Duct. 467 



" (2) The pronephros is situated within the segmented region 

 of the trunk, and has a more or less distinct metameric arrange- 

 ment of its parts ; while the provisional trochosphere organ is 

 placed in front of the segmented region of the trunk, and is in no 

 way segmented. 



" The comparison of the mesonephric tubules with the seg- 

 mented excretory organs of the Chsetopoda, though not impossible, 

 cannot be satisfactorily admitted till some light has been thrown 

 upon the loss of the supposed external openings of the tubes, and 

 the origin of their secondary connexion with the segmental duct." 



The difficulties concerning the phylogeny of the segmental 

 duct led Sedgwick (9) to the hypothesis that the duct may be com- 

 pared with "the circular canal of Medusae, which might easily be 

 conceived transformed into the Vertebrate segmental duct, the 

 excretory organs themselves being developed from the outer part 

 of the radial canals." At a more primitive stage in the evolution, 

 of Chordata he suggests that "the primitive alimentary canal 

 acquired a well-arranged system of ducts, by which the peripheral 

 ■excretory matters were carried to the part of the alimentary canal 

 near the hind end of the primitive mouth (future anus) ; that, in 

 consequence, the excretory pores [such as occur in the circular 

 oanal of Medusae] were not wanted, and were either never deve- 

 loped, or, if developed, lost." 



Sedgwick summarises his conclusions thus : — " With regard to 

 the endodermal organs, the pouches [archenteric diverticula] have 

 become differentiated into two kinds — 



" (1) Anteriorly a certain number retain their communication 

 with the exterior and with the gut. 



" (2) The majority, however, lose their connexion with the 

 gut and with the exterior, but remain connected by the peripheral 

 canal, which behind retains (by means of a pouch ?) its communi- 

 cation with the gut. 



" (3) A posterior pouch loses its connexion with the gut 

 and with the longitudinal canal, and gives rise to an abdominal 

 pore. 



" The first group of pouches become the gill-slits, the second 

 become the ccelom, while part of each of them become differentiated 

 into nephridia, which opens into the longitudinal canal (pronephric 

 or segmental duct). The last pair of pouches gives rise to a part 



