254 EMBRYOLOGY OF THE LOWER VERTEBRATES CH. 



with that which is found in the Anamnia, while the more highly 

 modified mode of development occurring posteriorly is equally readily 

 correlated with what happens in the metanephros of the Amniota. 



In the anterior region (approximately segments 12-15) the inter- 

 mediate cell-mass is compact, recognizably two layered, and the split 

 which separates the two layers may be obviously continuous with the 

 splanchnocoele (Fig. 137, A). It separates at an early stage from 

 the myotome, but it remains continuous at intervals with the lateral 



nt. 



nc. 



glom. 



p.c.v. 



-p.c.v. 



splc. 



P.C.U 



glom. 



Ki<;. 137. Sections illustrating the development of the mesonephros in Birds. (A and B, 

 after Sedgwick, 1881 ; C, D, E, after Schreiner, 1902). 



A, 22-segment chick at level of the 15th segment; B, 34-sr^niml chick at level ,,r \:\\\\ or itth 

 segment (combined from' two sections) ; C, 45-.se^nient duck at level ,,t -juti, segment : l>. ir>-s.--nu-nt 

 duck at level of 25tli segment ; K, 46-Mgmenl 'luck at level ( >f -Jlth segment. .1, dorsal aorta; n.n.il, 

 UfehinaphriC duet ; glom, fjlomerulus; nc, nephrocbele ; ./, nrplirotonie ; j>.<; peritoneal canal ; /i.e. r, 

 I>u>ti-iioi cardinal vein; sjilt; IplMlchnoCOele J t. (abide rudiment. 



mesoderm. The intermediate cell-mass becomes closely Jipjmsed to 

 and very soon directly continuous with the duct by a narrow isthmus 

 in each segment the tubule rudiment (Fig. 137, A). Ventrally, i.e. 

 near its jmn-tion with the splanchnocoele, the split between thr tun 

 layers of the nephrotome dilates and I'm-ius a definite m^jdinx ,!, 

 ujiicli nj.cii- iiiln the splanchnocoele by a wide peritoneal canal (Fig. 

 137, B,2?.c). The tubule rudiment develops a lumrn leading tYmi 

 inln duct. 1 and the dorsal wall of the nejlirnr<K'lc bc-nnu's 



1 Tin- .j.ihllii.in-iilirii- 'liK'l in tin- Ainnii.la ll kn..\\ n a> tin- iin-soiii-pliric or Wolffian 

 duct as its t'u ii'-t i<n i- ic -l i ic i, .| 1 1, drain in- the metOQephlOl OF " \V'l!li;iu IKH|V." 



