48 T. H. BURLEND, 



only (in the region of segments XIII to XX approx.) is the only 

 evidence against our regarding the duct as wholly derived from the 

 mesoderm. 



3. Is that part of the pronephric duct behind the pronephros to 

 be regarded as of the same nature as the duct in the region of 

 the pronephric tubules? 



Felix argues that this is the case in many Vertebrates, and 

 speaks of tubules and duct as the ,.Gesamtanlage" of the pronephros. 

 It is necessary only to trace the stages of development of the pro- 

 nephric duct in Chrijsemys to satisfy onesself that the pronephric 

 duct behind the region of segments IX and X of the pronephros 

 develops in exactly the same way as it does in these segments. There is 

 absolutely no break in the continuity of the parts which Felix calls 

 respectively ,.Sammelgang" and „mesodermale Endabschnitt" and so 

 the distinction is unnecessary. 



Thus we find: 



In front part of functional pronephros — (segments 

 VII and VIII) the kidney rudiment is a groove anteriorly and a 

 solid outgrowth from the somatic mesoderm behind: from this 

 rudiment both duct and tubules are simultaneously differentiated. 



In hinder region of functional pronephros (segments 

 IX, X, XI?, XII?) the duct seems to be a direct backward pro- 

 jection of the solid outgrowth; this is presumably reinforced from 

 the mesoderm ; later the duct fuses either completely or at intervals 

 with the mesoderm and from this fusion the hinder pronephric tubules 

 arise later. 



The development of the pronephric tubules in order, from before 

 backwards, is also found in other animals (Pristiurus, Myxinoids etc.). 

 In the region behind the pronephros the duct is formed first, the 

 tubules however sufter an arrested development (longer than in the 

 posterior pronephric region), with the result that when they do 

 appear they meet with a changed environment and become not 

 pronephric, but mesonephric, structures. 



The important factor in producing this changed environment in 

 the kidney region behind the pronephros is a rotation of the parts, 

 — outwards dorsally and inwards ventrally, resulting in: 



a) a relatively changed position of the duct, viz. from being 

 dorsal to the tubules it becomes ventro-lateral in position; 



b) an increase in length of the nephrotome region by the addition 



