196 OUTLINES OF CHORDATE DEVELOPMENT 



nearly filled by them. Each tubule carries around it a reflected 

 layer of the thin vascular wall and so is completely bathed in 

 the venous stream (Fig. 72). 



, 



ev 



FIG. 72. Sections through frog larvae illustrating the later development of 

 the pronephros. A. Through the first nephrostome of a larva of R. sylvatica of 

 about 8 mm., with prominent external gills. After Field. B. Through the region 

 of the second nephrostome of a 12 mm. larva of R. temporaries. After Fiirbringer. 

 c, Ccelom; cv, sinuses of posterior cardinal vein; eg, external glomerulus; g, gut 

 cavity; gX, ganglion nodosum (part of the ganglion of the vagus nerve) ; I, lung; 

 m, mesenchyme; myz, second myotome; p, peritoneum; si, ss, first and second 

 pronephric nephrostomes; tr, common trunk; X, root of vagus nerve. 



At the same time an arterial supply is derived from the 

 dorsal aorta. This is the pronephric glomus already mentioned. 





