78 T. H. BURLEND, 



evagination of the mesoderm of the lateral plate region. This primi- 

 tive condition is less evident further back (in the middle region of the 

 pronephros) where it assumes rather the form of a solid thickening 

 of the somatic mesoderm. From this rudiment arise by later modi- 

 fication both the anterior kidney tubules and the front part of the 

 prouephric duct. There is evidence that the "primitive kidney groove" 

 extended further forward in the ancestors of Chrysemys since remains 

 of this organ are found as far forward as segment III. 



2. The pronephric duct is of mesodermal origin, although contiguous 

 with the ectoderm for some distance in its middle region: inter- 

 segmental proliferations of mesoderm probably reinforce its back- 

 ward growth, though the rudiments of the duct appear before these 

 proliferations reach it in any particular region. 



3. The appearance of the duct precedes the appearance of the 

 posterior pronephric tubules by a short, and the anterior meso- 

 nephric tubules by a longer, interval. 



4. The pronephros attains its fullest development in segments 

 VII — X and probably extends into segments XI and XII (?) also. 



5. The pronephric duct passes into the gut in the region of the 

 primitive groove (blastopore). It becomes hollow from before back- 

 wards. 



6. The mesonephric tubules appear without any appreciable 

 break in time, and no break in space, behind the pronephric: 

 their difference in structure is entirely due to the different environ- 

 ment. 



7. The difference in the mesonephric rudiment (as compared 

 with the homologous pronephric) is due to a rotation of both duct 

 and kidney structures: this rotation through an angle of more 

 than 90° changes the position of the pronephric duct from a dorsal 

 to a lateral position relatively to the pronephros and also results in 

 a change in position of the mesonephric tubules so that they become 

 dorso-median with respect to the duct; the nephrostomes likewise 

 from being ventral and coelomic become median and nephrotomic. 

 This rotation is not of equal extent in the individual segments 

 being most complete in the middle of each segment, and less pro- 

 nounced at the ends. 



8. There is evidence that the original pronephric nephrostomes 

 divide up into a number of smaller ones and so tend to multiply 

 the number of nephrostomes to a tubule : this tendency in the meso- 

 nephric tubule is the probable cause of the so-called secondary, 



