Haddon — On the Epiblastic Origin of the Segmental Duct. 465 



somatic mesoblast, winch grows backwards between the epiblast 

 and the mesoblast (Cyclostomi, posterior portion ; Elasmobranchii ; 

 Amniota) . 



Balfour (1), appreciating the difficulties concerning the mor- 

 phology of the duct, wrote thus : — "It is quite certain that the 

 second of these processes is not a true record of the evolution of 

 the duct ; and though it is more possible that the process observable 

 in Amphibia and the Teleostei may afford some indications of the 

 manner in which the duct was established, this cannot be re- 

 garded as by any means certain." 



One question always presents itself : this is — How did the seg- 

 mental duct acquire its posterior connection with the cloaca ? In 

 the development of the duct this communication is effected later 

 than its first appearance, but this, evidently, could not represent 

 the ancestral condition. There are also several difficulties con- 

 cerning the general homology of the nephridia themselves. 



Balfour (1) discusses the problem in the following words: — 

 " It is a peculiarity in the development of the segmental tubes, 

 that they at first end blindly, though they subsequently grow till 

 they meet the segmental duct, with which they unite directly, 

 without the latter sending out any offshoot to meet them (Sedg- 

 wick maintains that the interior segmental tubes of the Chick form 

 an exception to this general statement). It is difficult to believe 

 that peritoneal infundibula ending blindly, and unprovided with 

 some external orifice, can have had an excretory function, and we 

 are therefore rather driven to suppose that the peritoneal infun- 

 dibula, which became the segmental tubes, were either from the 

 first provided each with an orifice opening to the exterior, or were 

 united with the segmental duct. If they were from the first pro- 

 vided with external openings, we may suppose that they became 

 secondarily attached to the duct of the pronephros (segmental 

 duct), and then lost their external openings, no trace of these 

 structures being left, even in the ontogeny of the system. It 

 would appear to me more fprobable that the pronephros, with its 

 duct opening into the cloaca, was the only excretory organ of the 

 unsegmented ancestors of the Chordata, and that, on the elonga- 

 tion of the trunk and its subsequent segmentation, a series of 

 metameric segmental tubes became evolved, opening into the seg- 

 mental duct, each^tube being in a sort of way serially homologous 



212 



