354 



EMBRYOLOGY. 



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 



Fig. 197. 



Fig. 198. 



Figs. 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. 



ch, 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 t 

 Will), parietal, visceral middle layer ; vn, pronephros ; 

 V 9i pronephric duct ; x, fissure in the primitive segment, 

 which is still in communication with the body-cavity. 



behind another, in Torpedo six 

 backwards and become united 



ranged one 

 which bend 

 cord. Soon afterwards the fundaments acquire small cavities 



vn) segmentally ar- 

 , in Pristiurus four, 

 into a longitudinal, 



