Haddon — On the Epiblastic Origin of the Segmental Duct. 469 



usually a marked segmental arrangement of the nephrie tubules in 

 ontology. The peripheral orifices of the nephridia must either 

 have opened directly to the exterior, or from the first debouched 

 into a longitudinal canal. Various theories have been framed to 

 explain the latter arrangement; but the former condition is un- 

 doubtedly more easily conceived, one difficulty in this supposition 

 being — What has become of the primitive external openings ? 



Accepting the proposition that the primitive Chordata ne- 

 phridia opened directly to the exterior, we have only to assume 

 that the lateral area along which they opened was grooved, and 

 that this groove extended posteriorly as far as the anus (Plate X.,. 

 figs. 3-5). 



From the analogy of the neural groove, there is no great diffi- 

 culty in further supposing that the nephrie groove was converted 

 into a canal, which, becoming separated from the overlying epi- 

 blast, might sink into the deeper-lying parts of the body. 



If a suggestion may be hazarded concerning the advantage of 

 converting the nephrie groove into the nephrie duct, it may be 

 pointed out that the lateral openings of the nephridia would not 

 be far removed from the branchial clefts, and the need of pure 

 water for respiratory purposes is emphasised by the now acknow- 

 ledged fact, that each cleft was provided with its own sense-organ 

 (now metamorphosed into the "thymus gland"). The develop- 

 ment of the duct from before backwards supports this view. 



From recent researches on the Lamprey [Shipley, 10], Newt 

 [Alice Johnson, 6], Alytes [Grasser, 4], and Frog [Spencer, 12], it 

 has been proved that in these forms the blastopore never closes 

 up, but persists as the anus [i.e. the opening of the mesenteron 

 into the cloaca). 



We are justified in assuming the persistence of the blastopore 

 as the anus in early Chordata : thus, if the nephrie groove were 

 continued round to the anus, it would practically open into the 

 extreme hinder end of the mesenteron, in other words, into the 

 urodseum [Gradow, 3]. 



Probably about the same time that the nephrie groove was 

 being converted into the nephrie canal (segmental duct) the proc- 

 todseum was being invaginated. The latter would push before it 

 the posterior orifice of the nephrie canal, as is represented in 

 Plate X., fig. 6. 



