354 



EMBRYOLOGY. 



Spff 



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. 



mp 



Fig. 198. 



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

 Pristiurus, after RABL. Cross section fig. 193 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- 

 notochonlal rod ; ao, aorta ; ik, inner germ-layer ; pmb, 

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

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

 which is still in communication with the body-cavity. 



ranged one behind 

 which bend backwards 

 cord. Soon afterwards 



vn) segment ally ar- 



another, in Torpedo six, in Pristiurus four, 

 and become united into a longitudinal 

 the fundaments acquire small cavities 



