388 s. H ATTA : 



Thanks to the labors of many eminent investigators, tlie 

 early development of tlie Selachian pronephros has been, as is well 

 known, fully studied, so that the facts gathered from this field 

 are well adapted to be compared with those from other groups. 

 I have found, in the present investigation, many important points 

 running parallel with the development of the Selachian pro- 

 nephros. I may then be allowed to compare my own results in 

 Petromyzon with those already arrived at in Selachia. Reference 

 will, however, be limited to those works which are sufficient to 

 verify the points I wish to bring out. 



Through the excellent work of Kuckert ('88) we can best 

 learn the origin of the pronephros in Selachia. " Die erste Anlage 

 der Vorniere " is recognised " in Form einer gegen den Ectoblast 

 gerichteten Vorbuch tung des parietalen Mesoblasts." This Anlage 

 is first brought about by the thickening of the parietal layer of 

 the mesoblast, which is found " in den Bereich des segmentirten 

 Mesoblasts, d.h. Somiten " (p. 209) ; this thickening is called by 

 the author '' Segmentalwulst." The foot-note also runs as follows : 

 " Der Ursprung des Segtnentalwulstes reicht ventral bis zu der 

 Stelle herab, wo die Somiten in den unsegmentirten Mesoblast 

 der Peritoneal wand übergehen " (p. 209). The " Segmentalwulst " 

 is so called because it is noticed as the segmental thicken- 

 ing of the parietal mesoblast of which E,uckert recognised, 

 in his Stad. ii^\ six for Toqjedo and four for Prisimrus, stretching 

 over a corresponding number of the myotomes. The first indication 

 of the pronephros is expressed, in Selachia also, segmentally in 

 the segmental part of the mesoblast at the stage in which the 

 metameric segmentation of the mesoblast is still going on, and 



l)Tlie embryos iu tlie stage have 25-27 somites. 



