The pronephros of Chrysemys margiuata. Q\ 



people to appreciate the exact meaning- of the term "nephrotome" or 

 "intermediate cell mass". Sedgwick was the first to discover this 

 structure, which he called (in the chick) the intermediate cell mass. 

 It consists of the somatic mesoderm and splanchnic mesoderm which 

 connect the protovertebrae (Ursegmente) with the mesodermal lining 

 of the general coelom (Seitenplatten). In a preliminary paper (22) I 

 stated that the kidney rudiment arises from the nephrotome in 

 Chrysemys, yet all my figures in that paper tend to show that the 

 pronephric part arises from the lateral plate mesoderm. This error 

 arose from the fact that I had not at the time examined rather 

 later stages with sufficient care, otherwise I should have seen that 

 the somatic mesoderm lateral to the anterior pronephric tubule 

 openings never becomes part of the nephrotome. I am indebted to 

 Prof Sedgwick for correction on this point. These two mesodermal 

 layers which form the nephrotome may or may not be separated by 

 a cavity ; in the anterior region of the body, where tlie metamerism 

 is well marked, the nephrotome is invariably a metameric structure. 



Moreover, the evidence that the pronephric tubules arise from 

 the lining of the general coelom is overwhelming. 



In Myxinoids the figures of Peice confirm this statement. 



Shipley and Goette independently observed this in the developing 

 lamprey. 



Balfour stated, before the nephrotome had been discovered, 

 that the pronephros arises from the lining of the coelom. 



Observations on the development of ScylUum canicula have con- 

 vinced me that this is the case in Selachii. 



Felix's figures for Teleosts and Ganoids; Wheelee's figures for 

 Fetromyson (1899); Semon's (1901) for Dipnoi; andMoLLiER's (1890) and 

 Field's (1891) work on Amphibia all point quite clearly to the lateral 

 plate origin of the pronephros. See also Kabl (1908) for lizard 

 and bird. 



Thus for Amphibia Felix writes (relying upon the work of 

 Field) : 



•'Auf dem Querschnitt scheint der Vornierenwulst in der ersten 

 Anlage aus 3 Zellenreihen zu bestehen, während das übrige Meso- 

 derm nur 2 Zellenreihen aufweist. Die höchste Erhebung des 

 Wulstes liegt etwas lateral von der Grenze zwischen sekundärem 

 Ursegment und Seitenplatte." . . . 



The later development of those cases such as Hypogeophis (Brauer, 

 1902), in which the tubules appear to arise from the nephrotome. 



