200 OUTLINES OF CHORDATE DEVELOPMENT 



merulus associated with each tubule, the two forming the 

 Malpighian body. Each glomerulus is connected with a small 



FIG. 74. Series of diagrams illustrating the development of the primary 

 mesonephric tubules in R. sylvatica. After Hall. The Wolffian duct is drawn 

 in outline simply. The mesonephric vesicles are shaded; the somatic part of 

 the tubule is shaded by continuous lines, the splanchnic part by dotted lines. 

 A. Wolffian duct and simple mesonephric vesicle. B. Mesonephric vesicle 

 dividing into the large primary mesonephric unit and the small dorsal chamber. 

 The latter elongates antero-posteriorly and represents the rudiment of the 

 secondary and later mesonephric units. C. Formation of the rudiment of the 

 inner tubule. D. Inner tubule extending upward and toward the mesonephric 

 duct; formation of rudiment of outer tubule. E. Outer tubule fused with peri- 

 toneum and rudiment of nephrostome thus established. Bowman's capsule form- 

 ing. Commencement of differentiation of secondary mesonephric tubules. F. 

 Separation of nephrostomal rudiment from remainder of tubule. G. Connection 

 cf nephrostome with branch of posterior cardinal vein. Separation of secondary 

 tubule, and beginning of tertiary tubule indicated, c, Bowman's capsule; i, inner 

 tubule; n, nephrostome; o, outer tubule; p, peritoneum; v, branch of posterior car- 

 dinal vein; /, primary mesonephric tubule; //, secondary mesonephric tubule; ///, 

 tertiary mesonephric tubule. 



twig derived from the dorsal aorta, and structurally resembles 

 essentially a miniature glomus like that of the pronephros. 



