DEVELOPMENT OF '1'H 1C V1C1NS IN THE EMBHYl) 1'IC. 11 



The advantages of the fixation in Carnov's mixture are that the .specimens arc even 

 clearer than after bleaching with peroxide, there are no bubbles formed to damage the 

 tissues, the time of the procedure is shortened, and the fixation is much better should it be 

 desired to section the specimens after studying the vessels in whole embryos. Specimens 

 which are strongly pigmented, however, must be bleached with hydrogen peroxide before 

 they can be cleared. 



GENERAL ANALYSIS OF THE PRIMITIVE VENOUS SYSTEM. 



It has been the universal opinion that the primary veins of the embryo, as distinct 

 from the veins of the embryonic membranes, are two. namely, the anterior and the posterior 

 cardinal. In recent years the idea has developed that the umbilical vein is in part a 

 vein of the membranes and in part a true vein of the body-wall, so that in this double 

 relation it differs from the omphalo-mesenteric veins. It was shown by Miss Helen 

 Smith, in 1909, that in the embryo pig, up to 10 mm. in length, all of the veins of the 

 body-wall ventral to the myotomes drain into the umbilical veins. Moreover, these 

 veins in the body-wall are non-segmental and have no accompanying arteries. In 1909 

 and 1912 H. M. Evans demonstrated that the primitive limb-bud veins drain not only 

 into the posterior cardinal vein, but into the umbilical vein as well, and thus it is fully 

 established that the umbilical vein is in part one of the primitive veins of the body-wall. 



It has been shown that in the chick the first vein of the embryo is the duct of Cuvier, 

 which develops as a direct cross-connection between the aorta and the vitelline veins and 

 that the anterior cardinal, the posterior cardinal, and the umbilical veins develop as a 

 further extension of the vascular arch between the aorta and the venous end of the heart. 

 (Evans, 1909; Sabin, 1915.) Moreover, in the pig a mesial cardinal vein, the so-called 

 subcardinal vein of F. T. Lewis, develops from the lateral branches of the aorta at the 

 same time and in the same manner as the posterior cardinal vein. On account of its 

 relation to the Wolffian body and to the posterior cardinal vein, the name mesial 

 cardinal vein seems to me better than subcardinal. It is therefore necessary to recast 

 the general statement in regard to the primitive veins of the mammalian embryo. 

 The primary venous arch of the embryo is the duct of Cuvier, which connects the aorta 

 with the vitelline veins and makes the first circle for blood within the embryo. Then, in 

 succession, four veins form which may be grouped as the primary venous system of the 

 embryo. First, an anterior cardinal vein by which the capillaries to the brain connect with 

 the duct of Cuvier and establish a circulation for the head; second and third, a posterior 

 and mesial cardinal vein from lateral branches of the aorta to the duct of Cuvier, developing 

 in relation to the Wolffian bodies; fourth, the umbilical veins as an arch between the aorta 

 and umbilical arteries, and the duct of Cuvier. It is clear that the anterior, posterior, and 

 mesial cardinal veins make one system and may be considered as a primary venous trunk 

 connecting with the heart through the duct of Cuvier and definitely accompanying the 

 aorta, while the umbilical vein is in part a vein of the membranes and in part a vein of the 

 body-wall, and is more remotely an accompanying vein to the aorta; that is to say, it forms 

 an arch which is more like the wide arch of the early circiilat ion of the embryonic membranes. 



The study of the development of the main veins of the body of the embryo consists. 

 then, in tracing how four veins the umbilical, the anterior, posterior, and mesial cardi- 

 nal veins develop, or disappear, or are transformed. 



Since the work of Goette in 1875, quoted from Hochstetter ( IS9H), it has been known 

 that the development of the vena cava in the liver was determined by a secondary connec- 

 tion of the cardinal svstein with the sinusoids of the liver. In 1901 02 this process was 



