. Carnegie Institution of Washington publication. FIG. 7.—Transverse section of an injected embryo pig of 30 somites, to show a typical cross-section of the vessels of the pronephros of the pig after the v. cardinalis mesialis has formed—that is, to show the pronephros with a cen- tral artery and three peripheral veins. The embryo measured 7 mm. after fixation and dehydration; it had no caudal flexure and was a little farther developed than the one on plate 5, figure 2. All the vessels shown were in- jected. The arteries are represented in black, the veins in white. The section is 50 p thick a
. Carnegie Institution of Washington publication. FIG. 7.—Transverse section of an injected embryo pig of 30 somites, to show a typical cross-section of the vessels of the pronephros of the pig after the v. cardinalis mesialis has formed—that is, to show the pronephros with a cen- tral artery and three peripheral veins. The embryo measured 7 mm. after fixation and dehydration; it had no caudal flexure and was a little farther developed than the one on plate 5, figure 2. All the vessels shown were in- jected. The arteries are represented in black, the veins in white. The section is 50 p thick and is unstained. X53. A. pr., artery of the pronephros which gives off capillaries to the tubules and extends to the v. cardinalis lateralis; V. c. L, v. cardinalis lateralis; V. c. m., v. cardi- nalis mesialis; V. c. p., v. cardinalis posterior; W. d., Wolf- fian FIG. 8.—Transverse section of an injected chick of 35 somites, after 69 hours of incubation, passing through the fifteenth somite. The section shows a typical cross- section of the vessels of the pronephros in the chick after the v. cardinalis mesialis has formed—that is, it shows the pronephros with a central artery and three peripheral veins. All the vessels were injected. The aorta is shown with a black rim, the artery is black, and the veins are white. The section is 50 /u thick and is unstained. X53. A. p., artery of the pronephros; V. c. I., v. cardinalis lateralis; V. c. m., v. cardinalis mesialis; V. c. p., v. car- dinalis posterior; W. d., Wolffian <lu<-t. to the nephritic tubules (text-fig. 7). At the stage of 30 somites a vein from this plexus passes ventral to the nephritic artery opposite the eleventh somite and grows caudalward just ventral to the nephritic arteries, between the aorta and the longitudinal artery of plate 3, figure 5. This is the medial cardinal vein, the subcardinal vein of F. T. Lewis. There is thus formed the primitive pattern of the circulation of the Wolffian bod
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