354 



EMBRYOLOGY. 



grows during larval life to an important organ, which disappears 

 after the animal's metamorphosis j 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- 

 Fig. 198. tion of the middle 



Figs 197 and 198. -Two cross sections through an embryo of germ - layer is COn- 



Fig. 107. 



pmk 



Pristiurus, after RAUL. Cross section fig. 193 lies a little 

 farther back than section fig. 197. 



with the 



tmuous 



ch, Chorda ; spg, spinal ganglion ; mp, muscle-plate of primitive lateral unsegmented 

 segment ; IF, skeletogenous tissue which has grown forth 



from the median wall of the primitive segment ; sck, sub- portion, there gl OW 



notochordal rod ; ao, aorta ; ik, inner germ-layer ; pmb, out of its parietal 



vmb, parietal, visceral middle layer ; vn, pronephros Jin i < 



*0, pronephric duct ; x, fissure in the primitive segment, lamella a number of 



v hich is still in communication with the body-cavity. ce n - 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 



