240 EMBEYOLOGY OF THE LOWEE VEETEBEATES oh. 



The independent backgrowth of the remainder of the duct in 

 Hypogeophis is probably to be regarded as a case of accelerated or 

 precocious development to allow the anterior tubules to become 

 functional at an early stage of development before those farther 

 back have developed. 



As regards the ontogenetic development of the main part of the 

 duct in other Vertebrates we find the most divergent statements 

 and it seems clear that this divergence can only be explained by 

 the actual facts not being always the same. 



In the Sauropsida it is admitted that the main part of the duct 

 is formed as in Hypogeophis by independent backgrowth. Amongst 

 the Anamnia the same is said to be the case in Elasmobranchs by 

 Balfour and by Eabl, and in Alytes according to Gasser, but other 

 authors describe two other methods of formation as occurring. 



The first of these is found in Elasmobranchs according to van 

 Wijhe, Beard, Eiickert and others. According to these investigators, 

 the archinephric duct makes its first appearance as a longitudinal 

 ridge - like thickening projecting inwards from the ectoderm. 

 This becomes split off as a solid ectodermal rod which develops 

 a cavity secondarily and forms the archinephric duct. Such a 

 mode of development would be of great morphological interest 

 as it would lend decided support to the view that the archinephric 

 duct originated in evolution as an ectodermal groove — it being a 

 common ontogenetic modification that what is morphologically a 

 groove develops ontogenetically in the form of a solid ridge-like 

 ingrowth. It has however to be borne in mind that there exists a 

 serious source of possible error in making observations upon the archi- 

 nephric duct in early stages. The duct lies between ectoderm and 

 somatic mesoderm — the two cell- layers mentioned fitting close round 

 it. During the various processes to which the embryo is subjected 

 preparatory to being cut into sections the ectoderm usually separates 

 slightly from the mesoderm, and the archinephric duct tends to 

 adhere firmly to one or other of these layers. This is the case more 

 particularly at its tip, where it is pushing the ectoderm and meso- 

 derm apart as it grows back and is therefore in particularly intimate 

 contact with them. It is exceedingly difficult in studying sections 

 to distinguish with certainty between such intimate contact and actual 

 organic continuity. In cases where the hinder part of the duct is 

 adherent to the ectoderm an appearance is produced which simulates 

 closely a development by splitting off from the ectoderm. 



As a matter of fact C. Babl's very careful investigations (1896) 

 fail to confirm the ectodermal origin of the duct in Elasmobranchs 

 and upon the whole in the writer's opinion there does not appear to 

 be any longer justification for accepting it as actually occurring. 



The other mode, by which the extension of the archinephric duct 

 backwards has been described as taking place in the Anamnia is 

 that the duct becomes split off from the underlying somatic meso- 

 derm. It is necessary again to bear in mind the caution expressed 



