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 vat li 

 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 unst'gmented 

 portion, there grow 

 out of its parietal 

 lamella a number of 



Fig 107. 



Fig. 198. 



Fiji 197 and 198. Two CTOM sections through an embryo of 

 Pristiunu, after RABL. Cross section fig. 193 lies a little 

 farther back than section rig. 197. 



<7, Chorda; ipg, spinal ganglion ; mp, muacle-plate of primitive 

 tegment ; W, skeletogenous tiaeue which has grown forth 

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

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

 VM&, parietal, visceral middle layer; vn, pronephros; 

 rg, pronephriu duct ; x, fissure in the primitive segment, 

 ukich is still in communication with the body-cavity. 



cell -cords (fig. 197 

 vn) segment ally ar- 

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

 which b-nd backwards and become united into a longitudinal 

 cord. Soon afterwards the fundaments acquire small cavities 



