228 EMBRYOLOGY OF THE LOWER VERTEBRATES CH. 



126. Tian >nt of /V/////v/-//.s stairs 20, 23, 



ami 'JH jas>iiiK Iliron^'li tin- nnllim-nl ( n<-pln osluinr 

 B, which ift seen )irnj-i-tiii;r out wants from tin- wall of 

 111.- iicpin-' 



,il aorta; '''/, colmii<- MVlfe) : M6, uiyoooela ; 



; .HirliMal \.-il, ; 



split, re].' 



In other Verte- 

 1 rates possessing a 

 functional pronephros 

 the appearances seen 

 in early stages are 

 readily reconcilable 

 with those described 

 above for Hypogeo- 

 phis, and we may 

 take it that, apart 

 from variations in 

 detail, this represents 

 the normal mode of 

 development of the 

 organ. 



CROSS OPT ERYGII. 



- In % Crossoptery- 

 gians, so far at least 

 as Polypterus the 

 only member of the 

 group investigated 

 is concerned, the first 

 rudiments of the pro- 

 nephric tubules are 

 in the form of pro- 

 jections which pass 

 outwards and back- 

 wards from the exter- 

 nal side of each of the 

 anterior nephrotomes 

 (Fig. 125, A). The 

 number of these 

 tubule rudiments pre- 

 simiiilily varies, seven 

 being seen in one 

 specimen and nine in 

 another. Apparently 

 the tubule rudiments 

 become fused at their 

 outer ends to form a 

 solid mass the rudi- 

 ment of the aivhi- 

 ncj.liric duct. In the 

 stages shown in V\g. 

 L26, A and B, five 

 lulmlo rudiments are 

 si-en passing ;it their 

 outer ends into the 



