354 



EMBRYOLOGY. 



grows during larval life to an important organ, which disappears 

 after the ammal's metamorphosis ; finally, in the Selachians and 



Amniota its funda- 

 ment is from the 

 beginning very rudi- 

 mentary. In the 

 latter case it was 

 held to be the front 

 end of the meso- 

 nephric duct, until 

 through comparative 

 embryology the right 

 view had been at- 

 tained. 



I select as types 

 of the development 

 of the pronephros 

 the Selachians, Am- 

 phibia, and Birds. 



In Selachians of 

 about twenty -seven 

 somites the prone- 

 phros begins with 

 the third or fourth 

 trunk - segment and 

 is developed from 

 there backwards. 

 At the place where 

 the segmented por- 

 tion of the middle 

 germ - layer is con- 

 tinuous with the 

 lateral unsegmented 

 portion, there grow 

 out of its parietal 

 lamella a number of 

 cell -cords (fig. 197 

 vn) segmentally ar- 

 in Torpedo six, in Pristiurus four, 

 become united into a longitudinal 

 fundaments acquire small cavities 



Fig. 197. 



Fig. IDS. 



Figs. 197andl96» — Two cross sections through an emhryo of 

 Pristiurus, after Rabl, Cross secbioa fig. 193 lies a little 

 farther back than section fig. 197. 



eh, Chorda ; «pf/, spinal ganglion ; wip, muscle-plate of primitive 

 segment ; W, skeletogenous tissue which has grown forth 

 from the median wall of the primitive segment ; sch, sub- 

 notochordal rod ; ao, aorta ; ik, inner germ-layer ; puift. 

 vmh, parietal, visceral middle layer ; vn, pronephros ; 

 vg, pronephric duct ; x, fissure in the primitive segment, 

 which is still in communication with the body-cavity. 



ranged one behind another, 

 which bend backwards and 

 cord. Soon afterwards the 



