32 EXPLANATION OF PLATE.S. 



PLATE 4. 



FIG. 9. Dissection of an embryo pig 16 mm. long in which the vascular system has been injected with India ink 

 through the umbilical artery. The specimen shows the prevertebral plexus of the cervical and upper 

 dorsal region with the developing v. hemiazygos. X1S. D. c., ductus Cuvier; v. HA., v. hemiazygos; 

 v. J. K., v. jugularis externa; v. TE., v. thoraco-epigastrica. 



10. Transverse section of an embryo pig 16 mm. long in which the vascular system has been injected with India 

 ink through the umbilical artery. The section passes through the cephalic pole of the Wolffian body and 

 is to show the relation of the v. azygos to the prevertebral plexus. The section is unstained and is 

 2. r iO /u thick. It is about the stage of fig. 13. X34. A., aorta; v. c. p., v. cardinalis posterior within 

 the Wolffian body; v. HA., v. hemiazygos; v. M. A., v. mesialis anterior of the Wolffian body; v. M. I., 

 v. mammaria interna; v. TE., v. thoraco-epigastrica; v. v., v. ventralis of the Wolrnan body; w. B., 

 Wolffian bodv. 



11. Dissection of an embryo pig 20 mm. long in which the vascular system has been injected with India ink 



through the umbilical vein. All of the viscera have been removed. X20. A., aorta; v. A., v. azygos; 

 v. B., common stem of the v. fibularis primitiva and the v. caudalis which becomes the lower segment 

 of the vena cava; v. o. p., v. cardinalis posterior; v. L. T., v. lumbalis transversa; v. s. 2, v. spinalis 

 of the second segment; v. TE., v. thoraco-epigastrica; v. u. P., v. ulnaris primitiva. 



12. Transverse section of an embryo pig 20 mm. long in which the vascular system has been injected with India 



ink through the umbilical artery. The section is to show the prevertebral plexus and passes through 

 the median mesonephritic vein. The section is unstained and is 250 n thick. X77. A., aorta; A. s., 

 a. spinalis, which is now a branch of a single dorsal segmental artery; p. P., prevertebral plexus at the 

 position of the future azygos vein, the limit of the development of the v. azygos for this stage being shown 

 in fig. 11; p. p'., prevertebral plexus at the point in which the third segment of the inferior vena cava 

 and the corresponding ascending lumbar vein develop at the level below the median mesonephritic vein; 

 p. p. v., prevertebral plexus of small veins resting on the vertebra; v. M. M., v. mediana mesonephritica, 

 the anastomosis across the mid-line of the embryo of the two vv. cardinalis mesiales; v. T. M., v. trans- 

 versa mesialis of the Wolffian body. 



PLATE 6. 



Hi. Dissection of an embryo pig 17 mm. long in which the vascular system has been injected with India ink 

 through the umbilical artery. X21. A., aorta; D. c., ductus Cuvier; p. p., prevertebral plexus which 

 receives veins from the body-wall and from the spinal cord and drains into the Wolffian body in a sagittal 

 plane lateral to the hemiazygos vein; v. HA., v. hemiazygos; v. M. I., v. mammaria interna; v. TE., 

 v. thoraco-epigastrica. 



1 t. Diagram from the embryo pig measuring 24 mm. long to show the relations of the symmetrical segments 

 of the inferior vena cava to the median mesonephritic vein and to the remnants of the posterior cardinal 

 vein in stages measuring from 20 to 27 mm. long. The parts of the large median vein marked with a cross 

 are from the posterior cardinal vein. The dotted lines separate the part of the median vein which per- 

 sists from that which disappears with the Wolffian body. A., aorta; K., kidney; v. c., v. caudalis; 

 v.c.i., vena cava inferior; v.c. i. L., vena cava inferior, lower segment; v. F. p., v. fibularis primitiva; 

 v. M. M., v. mediana mesonephritica; V.T..V. transversa of the Wolffian body. 



15. Dissection of an embryo pig 20 mm. long, a little younger than the specimen of figs. 4 and ."i. The specimen 



has been injected with silver nitrate through the veins and is to show the stage at which the v. azygos 

 and the v. cardinalis posterior are equal in size and both empty into the duct of Cuvier. XS. D. c., 

 ductus Cuvier; v. A., v. azygos; v. c. A., v. cardinalis anterior or v. jugularis interna ; v. c. I., vena cava 

 inferior; v. c. P., v. cardinalis posterior; v. j. E., v. jugularis externa; v. TE., v. thoraco-epigastrica; 

 w. B., Wolffian body. 



PLATE 7. 



16. Injection of the veins in an embryo pig 28 mm. long to show the relation of the v. hemiazygos accessoria to 



the caudal part of the v. cardinalis sinistra which still persists at this stage. X17. D. c., ductus 

 Cuvier; v. A., v. azygos; v. c. A., v. cardinalis anterior; v. c. P., v. cardinalis posterior; v. HA., v. hemi- 

 azygos; v. HA. A., v. hemiazygos accessoria; v. o., oblique vein which is a branch of the v. azygos; 

 \v u ., Wolffian body. 



17. Dissection of an embryo pig is mm. long in which the vascular system has been injected with India ink 



through the umbilical artery. The specimen shows the azygos and hemiazygos veins and their relation 

 tn the disappearing vv. cardinales posteriores. X20. v. A., v. azygos; v. c., common stem of the 

 v. ulnaris primitiva and the v. thoraco-epigastrica; v. c. A., v. cardinalis anterior; v. c. P., v. cardinalis 

 posterior; v. HA. A., v. hemiazygos accessoria; v. o., oblique vein which is a branch of the v. azygos; 

 w. B., Wolffian body. 



18. Dissection of an embryo pig 40 mm. long in which the vascular system has been injected with India ink 



through the umbilical artery. View of the v. azygos and v. hemiazygos. X26. v. A., v. azygos; 

 v. HA., v. hemiazygos; v. HA. A., v. hemiazygos accessoria; v. o., oblique vein which is a branch of 

 the v. azygos. 



32 



