64 T. H. BURLEND, 



structure developed from the somatic mesoderm — is the first rudi- 

 ment of the pronephros? 



No doubt Felix's wide knowledge of the pronephros led him to 

 record his observations on Teleosts after very careful confirmation, 

 but 1 think the exceptional nature of his results requires further 

 work on the Teleosts. 



There is no need to adduce proof of the cranio-caudal direction 

 of pronephros growth. In Myxinoids onwards this method is not 

 deviated from. But if — as most authorities agree — the pronephros 

 was formerly much more extensive in Vertebrates (Rückert, Wieders- 

 HEiM etc.) the statement that its growth takes place in this direction 

 is paramount to admitting that the change which goes on whereby 

 the typical pronephros is shortened and its nature disguised, is a 

 change which proceeds from behind forwards. It is this important 

 process of change, which aifects kidney development in such a way 

 as to cause a rotation of the kidney duct from a dorsal to a lateral 

 position, and causes the pronephric tubule nephrostomes to shift from 

 a lateral to a median position owing to an increase in the extent 

 of the nephrotome at the expense of the general coelom, that I shall 

 discuss in detail later. 



The observation that the pronephric duct is formed before the 

 posterior pronephric tubules, is a proof that we cannot distinguish 

 between pronephric and mesonephric tubules by saying that the 

 former open into the duct from the start and the mesonephric open 

 in secondarily. The development of the pronephric duct in Chrysemys 

 precedes the development and differentiation of the posterior pro- 

 nephric tubules, and to a much greater extent precedes the establish- 

 ment of the mesonephric tubules. 



The time and order of development of the tubules are similar 

 in Myxinoids {Bdellostoma Price), Chrysemys etc. 



With regard to the caudo-cranial process. Felix, quoting Price, 

 writes : 



"Bei den älteren Embryonen (Stadium B von Price) ist die 

 Loslösung der Vornierenkanälchen vom Cölom bis zum 29. Segment 

 kranialwärts vorgeschritten." 



Again, with regard to the development of the Trout embryo 

 Felix writes: 



"Erstens löst sich die primäre Vornierenfalte im Bereiche des 

 6. — 7. Ursegments von der Seitenplatte' ab, der abgelöste Teil setzt 



