354 



EMBRYOLOGY. 



*P9 



grows during larval life to an important organ, which disappears 

 after the animal'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- 

 ranged one behind another, in Torpedo six, in Pristiurus four, 

 which bend backwards and become united into a longitudinal 

 cord. Soon afterwards the fundaments acquire small cavities 



Fig. 197. 



Fig. 198. 



Tigs. 197 and 198. Two cross sections through an embryo of 

 Pristiurus, after RABL. Cross section fig. 198 lies a little 

 farther back than section fig. 197. 



fli, Chorda ; spg, spinal ganglion ; mp, 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 : pmb, 

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

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

 which is still in communication with the body-cavity. 



