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. 



ric. 



'A 



■p.c.-o. 



it-., y -t * ' 



glom 



Fig. 



137. — Sections illustrating the development of the mesonephros in Birds. (A and B, 

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



A, 22-segment chick at level of the 15th segment ; B, 34-segment chick at level of 13th or 14th 

 segment (combined from two sections) ; C, 45-segment duck at level of 29th segment ; D, 45-segment 

 duck at level of 25th segment ; B, 45-segment duck at level of 24th segment. A, dorsal aorta ; a.n.d, 

 archinephric duct ; glom, glomerulus; wc, nephrocoele ; nt, nephrotome ; p.c, peritoneal canal ; p.c.v, 

 posterior cardinal vein ; splc, splanchnocoele ; t^ tubule rudiment. 



mesoderm. The intermediate cell-mass becomes closely apposed 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 junction with the splanchnocoele, the split between the two 

 layers of the nephrotome dilates and forms a definite nephrocoele 

 which opens into the splanchnocoele by a wide peritoneal canal (Fig. 

 137, B, p.c). The tubule rudiment develops a lumen leading from 

 nephrocoele into duct, 1 and the dorsal wall of the nephrocoele becomes 



1 The opisthonephric duct in the Amniota is known as the mesonephric or Wolffian 

 duct as its function is restricted to draining the mesonephros or " Wolffian body." 



