356 OUTLINES OF CHORDATE DEVELOPMENT 



duced to small transitory buds entirely disappearing during the 

 latter part of the second day. 



Soon after the tubule appears in each segment, the nephro- 

 tome is separated from the somite by the conversion of its 

 proximal part into mesenchyme, and the distal part then 

 appears added to the tubule. The only cavity of the pro- 

 nephric tubule is one sometimes appearing in this added por- 

 tion of the nephrotome, and is to be regarded as a continuation 

 of the coelom of the lateral plate into the nephrotome region; 

 when present its opening to the coelom would therefore repre- 

 sent a nephrostome. No Malpighian bodies are developed in 

 connection with these tubules, and the whole pronephros 

 disappears by the end of the third day. 



B. THE MESONEPHROS 



The mesonephros is the functional embryonic kidney; its 

 duct is the original pronephric or Wolffian duct. The meso- 

 nephros begins to develop toward the end of the second day in 

 the region immediately posterior to the pronephros. Meso- 

 nephric tubules finally develop in all segments from the thir- 

 teenth or fourteenth to the thirtieth; the most anterior tubules 

 are thus present in segments developing pronephric tubules also. 

 In front of the twentieth segment, however, the mesonephros 

 remains rather vestigial and develops typically only from the 

 twentieth to the thirtieth segments. 



In this latter region the narrow nephrotomal band widens, 

 separates entirely from the lateral plate and the somites, and 

 the original arrangement of its cells in dorsal and ventral layers 

 is lost. We should note that the Wolffian duct passes along, 

 between the nephrotomes and the somatic layer of the lateral 

 plate, w r hile along the opposite sides of the nephrotomes is the 

 dorsal aorta; the posterior cardinal veins soon appear just 

 above the Wolffian ducts. On the ventral side of the nephro- 

 tome, about opposite the middle of the segment, its cells become 

 condensed into a spherical mass, in which a definite space 

 appears; this is the rudiment of the primary mesonephric tubule 



