228 EMBRYOLOGY OF THE LOWEE VERTEBRATES ch. 



In other Verte- 

 brates possessing a 

 functional pronephros 

 the appearances seen 

 in early stages are 

 readily reconcilable 

 with those described 

 above for Kypogeo- 

 phis, and we may 

 take it that, apart 

 from variations in 

 detail, this represents 

 the normal mode of 

 development of the 

 organ. 



C BOSS 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- 

 sumably 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 archi- 



Fig. 125.— Transverse sections of Polypterus— stages 20, 23, nephric duct. In the 



and 28— passing through the rudiment of nephrostome stages shown in Fig. 



B, which is seen projecting outwards from the wall of -,r,r. « and B five 



the nephroeoele. > ' • 



tubule rudiments are 



A, dorsal aorta ; C'oeZ, coelome ; ent, enteric cavity ; me, myocoele ; ggg n fjaSSinS' at their 

 N, notochord ; nc. nephroeoele ; p.c.v, posterior cardinal vein ; splc, ^ 5 _ , 



split representing splanchnocoele. Ollter ends into trie 



